minigbm: Added YV12 for Mali platforms

minigbm: Added YV12 for Mali platforms

Oak-cheets is using YV12 as it's Android flexible YUV format, so
we need to support it.  Additionally, we're trying to enable
YV12 on Mali with EXT_image_dma_buf_import.  Instead of having
redundant switch statements in every driver, let's add a helper
function to do this.

BUG=chrome-os-partner:54632, b/29059119, chromium:616275
TEST=Ran plane_test, graphics_Gbm on veyron_minnie

Change-Id: I4798225db809941367e58dde962576535b8d767c
Reviewed-on: https://chromium-review.googlesource.com/377884
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>