minigbm: Fix the flag situation

minigbm: Fix the flag situation

FrameBufferSurface.cpp (used by SurfaceFlinger) requires
GRALLOC_USAGE_HW_FB and GRALLOC_USAGE_HW_COMPOSER flags.

It's annoying to update so many flags in driver-supported lists, and
difficult to read.  Let's remove redundant flags and try to keep the
ones that one have a distinct and granular meaning.

CQ-DEPEND=CL:362062

Change-Id: I6e1974b02e90a4a98c350867d71050fee728ce3a
Reviewed-on: https://chromium-review.googlesource.com/392546
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>