The following tests began failing with mesa cda886a4851ab767fba40e8474d6fa8190347e4f:

deqp-gles2.functional.texture.completeness.2d.format_mismatch_rgb_rgba 
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgba_rgb_level_0_neg_z 
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgba_rgb_level_0 
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgb_rgba_level_0_pos_z 
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgb_rgba_level_0 
deqp-gles2.functional.texture.completeness.2d.format_mismatch_rgba_rgb 

Sample output:
/tmp/build_root/m64/opt/deqp/modules/gles2/deqp-gles2 --deqp-case=dEQP-GLES2.functional.texture.completeness.cube.format_mismatch_rgb_rgba_level_0 --deqp-surface-type=fbo --deqp-log-images=disable --deqp-surface-width=256 --deqp-surface-height=256
dEQP Core 2014.x (0xcafebabe) starting..
  target implementation = 'DRM'

Test case 'dEQP-GLES2.functional.texture.completeness.cube.format_mismatch_rgb_rgba_level_0'..
Test case duration in microseconds = 16674 us
  Fail (Image comparison failed)

DONE!

Test run totals:
  Passed:        0/1 (0.0%)
  Failed:        1/1 (100.0%)
  Not supported: 0/1 (0.0%)
  Warnings:      0/1 (0.0%)

Of the 6 tests, two were previously asserting, and do not constitute a regression:
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgba_rgb_level_0_neg_z
deqp-gles2.functional.texture.completeness.cube.format_mismatch_rgb_rgba_level_0_pos_z

The bisected patch:
Author:     Neil Roberts <neil@linux.intel.com>
AuthorDate: Thu Nov 19 16:25:21 2015 +0100
Commit:     Neil Roberts <neil@linux.intel.com>
CommitDate: Wed Jan 13 12:16:31 2016 +0000

    i965/gen9: Don't allow the RGBX formats for texturing/rendering
    
    The RGBX surface formats aren't renderable so we internally remap them
    to RGBA when rendering. They are retained as RGBX when used as
    textures. However since the previous patch fast clears are disabled
    for surfaces that use a different format for rendering than for
    texturing. To avoid this situation we can just pretend not to support
    RGBX formats at all. This will cause the upper layers of mesa to pick
    an RGBA format internally instead. This should be safe because we
    always override the alpha component to 1.0 for RGBX in the texture
    swizzle anyway. We could also do this for all gens except that it's a
    bit more difficult when the hardware doesn't support texture
    swizzling. Gens using the blorp have further problems because that
    doesn't implement this swizzle override.
    
    Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>