r600g: fast color clears for single-sample buffers

r600g: fast color clears for single-sample buffers

Allocate a CMASK on demand and use it to fast clear single-sample
colorbuffers. Both FBOs and window system colorbuffers are fast
cleared. Expand as needed when colorbuffers are mapped or displayed
on screen.

v2: cosmetics, move transfer expansion into dma_blit

Signed-off-by: Marek Olšák <marek.olsak@amd.com>