minigbm: DRM_FORMAT_BGR888 for amdgpu

minigbm: DRM_FORMAT_BGR888 for amdgpu

Move DRM_FORMAT_BGR888 from render_target_formats to texture_source_formats

BUG=b:110472317
TEST=test_that --board=grunt graphics_Gbm

Change-Id: I4da4ca1544b0596ef39c24f268c85014ef58ad7e
Reviewed-on: https://chromium-review.googlesource.com/1108756
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>