ANDROID: drm/virtio: implement context init: track valid capabilities in a mask

The valid capability IDs are between 1 to 31, and defined in the
virtio gpu spec.  This is used for error checking the subsequent
patches.  We're currently only using 2 capability IDs, so this
should be plenty for the immediate future.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>

BUG: 173630595
TEST: boot VM

Change-Id: I86cc131201ac0abb0f99bb6d25ae86b508beb610