Various cleanups for gfx_v8_0.c

[Tom St Denis]

These patches perform various cleanups in the GFX v8 driver. 

Patch #1, performs various whitespace/error message cleanups.  

Patch #2, performs various simplifications to routines to make them
easier to read (and reduce LOC).

Patch #3, introduces a WREG32_FIELD() macro which is then used to
reduce various patterns of setting fields in mmio registers down 
to a single line.