minigbm: rockchip/mediatek: remove R8 as texture source on Mali/Bifrost

minigbm: rockchip/mediatek: remove R8 as texture source on Mali/Bifrost

EGL can't import this format on Mali/Bifrost.

BUG=chromium:969044
TEST=Ozone unit test

Change-Id: I356ec2bd35030af600b268bf39565e30e16465f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1643677
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Commit-Queue: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>