minigbm: Remove SCANOUT flags from YV12

minigbm: Remove SCANOUT flags from YV12

All of these drivers don't support the ability to directly scan-out
YV12 to the display, so this should not be set.
Let's set rendering flags since the expected use case is importing
Android YV12 buffers into EGL.

BUG=b/32260847
TEST=AdaptivePlaybackTest#testH264_adaptiveDrc

Change-Id: I53d4eaf9c3f359c1c83762cf3e655cc0cf5168d5
Reviewed-on: https://chromium-review.googlesource.com/401040
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>