minigbm: add GBM_BO_USE_SW_{READ,WRITE}_{OFTEN,RARELY} usage flags

minigbm: add GBM_BO_USE_SW_{READ,WRITE}_{OFTEN,RARELY} usage flags

Need for these flags has arisen because buffer's allocated
BO_USE_SCANOUT flag is not mmapable in amd, whereas going further
for ensuring atomictity related operations are functional, one
needs buffer to be mmap'd.

These flags shall be used to strike balance in enabling
AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED on amd and mmap X-tiled buffers
on Intel platforms.

BUG=b:65297611, chromium:777706
TEST=On Kahlee,
     UI comes up.
     GLMark2 & GLbench autotests show no performance regression
     atomictest -t multiplanes passes
     WebGL Aquarium

Change-Id: I582452d331f4a05106939447784a76eba06d13c2
Reviewed-on: https://chromium-review.googlesource.com/758618
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>