graphics: Fix GXPix* assignment

graphics: Fix GXPix* assignment

We need assign cols/rows to GXPixCols/GXPixRows separately, we can't
rely on the compiler writing the 32-bit value to two consecutive
16-bit memory locations like the assembly version did. The value we
were storing was actually being truncated by the compiler (see the
warning below),

Also fixup the following warnings,

graphics.c: In function ‘vgasetmode’:
graphics.c:95:18: warning: cast from pointer to integer of different size
graphics.c:102:2: warning: large integer implicitly truncated to unsigned type
graphics.c: In function ‘outputvga’:
graphics.c:222:12: warning: cast from pointer to integer of different size
graphics.c:227:11: warning: cast from pointer to integer of different size
graphics.c: In function ‘vgadisplayfile’:
graphics.c:303:4: warning: passing argument 1 of ‘outputvga’ from incompatible pointer type
graphics.c:213:13: note: expected ‘uint32_t *’ but argument is of type ‘uint8_t *’

Signed-off-by: Matt Fleming <matt.fleming@intel.com>