Bug 2924

These drivers have some questionable pointer casting that causes GCC to generate
warnings on x86-64 systems.  I looked at a couple cases in the Unichrome driver,
and they appear to be things like this:

vmesa->regEngineStatus = (GLuint *)((GLuint)viaScreen->reg + 0x400);

That line actually generates *2* warnings.  One is for the cast from drmAddress
(viaScreen->reg) to a GLuint, and the other is for the cast from GLuint to GLuint*.

I don't have any hardware that I can put in my x86-64 box to "verify" a fix to
this.  However, it seems like the right fix is to replace
'(GLuint)viaScreen->reg + 0x400' with '(GLubyte *)viaScreen->reg + 0x400'.  That
should generate the right result without the funky pointer-to-int casting.