minigbm: Add mediatek drm to minigbm

Add gbm_driver_mediatek function in gbm_get_driver to support
mediatek drm

BUG=None
TEST=Use gbmtest with minigbm to verify

Change-Id: Ieaccdbf5a2bde94f70addb8ac5192fbbcecf321e
Signed-off-by: JB Tsai <jb.tsai@mediatek.com>
Reviewed-on: https://chromium-review.googlesource.com/261503
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
2 files changed