Bisected to

commit 18442d08786472c63a0a80c27f92b033dffc26de
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Sep 13 16:00:08 2013 +0300

    drm/i915: Fix port_clock and adjusted_mode.clock readout all over
    
    Now that adjusted_mode.clock no longer contains the pixel_multiplier, we
    can kill the get_clock() callback and instead do the clock readout
    in get_pipe_config().
    
    Also i9xx_crtc_clock_get() can now extract the frequency of the PCH
    DPLL, so use it to populate port_clock accurately for PCH encoders.
    For DP in port A the encoder is still responsible for filling in
    port_clock. The FDI adjusted_mode.clock extraction is kept in place
    for some extra sanity checking, but we no longer need to pretend it's
    also the port_clock.
    
    In the encoder get_config() functions fill out adjusted_mode.clock
    based on port_clock and other details such as the DP M/N values,
    HDMI 12bpc and SDVO pixel_multiplier. For PCH encoders we will then
    do an extra sanity check to make sure the dotclock we derived from
    the FDI configuratiuon matches the one we derive from port_clock.
    
    DVO doesn't exist on PCH platforms, so it doesn't need to anything
    but assign adjusted_mode.clock=port_clock. And DDI is HSW only, so
    none of the changes apply there.
    
    v2: Use hdmi_reg color format to detect 12bpc HDMI case
    v3: Set adjusted_mode.clock for LVDS too
    v4: Rename ironlake_crtc_clock_get to ironlake_pch_clock_get,
        eliminate the useless link_freq variable.
    
    Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
    Reviewed-by: Jani Nikula <jani.nikula@intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>