minigbm: align width so that stride aligns to 256

minigbm: align width so that stride aligns to 256

map stride is bigger than the allocation stride. Currently
gralloc_lock does not consider map_stride. Align the width so
that stride get alinged to 256.

BUG=b:115946221
TEST= CtsNativeHardwareTestCases GpuColorOutputCpuRead* tests

Change-Id: I0e1ccfba4ec981702498a76fa5a0b2c662b6e728
Signed-off-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-on: https://chromium-review.googlesource.com/1242767
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Deepak Sharma <deepak.sharma@amd.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>