minigbm: amdgpu: align the stride to 256

minigbm: amdgpu: align the stride to 256

amdgpu has a requirement of stride to be aligned to 256.
Otherwise gpu faults and corruptions are observed for some
resolutions.

BUG=b:80148696 b:110472790
TEST=graphics autotest suite

Change-Id: Ic13c19cd1641a6ce206de9b1016a242ed21c2631
Signed-off-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-on: https://chromium-review.googlesource.com/1133060
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Deepak Sharma <deepak.sharma@amd.com>
Reviewed-by: Bas Nieuwenhuizen <basni@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Deepak Sharma <deepak.sharma@amd.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>