mesa: update packed format layout comments

Update the comments for the packed formats to accurately reflect the
layout of the bits in the pixel.  For example, for the packed format
MESA_FORMAT_R8G8B8A8, R is in the least significant position while A
is in the most-significant position of the 32-bit word.

v2: also fix MESA_FORMAT_A1B5G5R5_UNORM, per Roland.
1 file changed