r600g: rewrite FMASK allocation, fix FMASK texturing with 2 and 4 samples

r600g: rewrite FMASK allocation, fix FMASK texturing with 2 and 4 samples

This fixes and enables texturing with compressed MSAA colorbuffers
on Evergreen and Cayman. For the first time, multisample textures work
on Cayman.

This requires the libdrm flag RADEON_SURF_FMASK.

v2: require libdrm_radeon 2.4.45

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>