minigbm: cros_gralloc: map protected flag to linear

minigbm: cros_gralloc: map protected flag to linear

This should be fine since the dummy fd was never used
with the encoder / decoders, so it doesn't need to be
tiled.

BUG=b/171018855
TEST=GtsExoPlayerTestCases / GtsMediaTestCases

Change-Id: I1309a641e19a9196e0aed05f4820e0d2672fafa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/2481223
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: David Stevens <stevensd@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Auto-Submit: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: Gurchetan Singh <gurchetansingh@chromium.org>