drm_hwcomposer: Check against gem handles for popped buffer

drm_hwcomposer: Check against gem handles for popped buffer

Add check before closing gem handle for most recently popped buffer in
addition to buffers in queue.

Change-Id: Icb096bd4db439b1195a123e28468142829fc61ea
Reviewed-on: https://chrome-internal-review.googlesource.com/200725
Reviewed-by: Stéphane Marchesin <marcheu@google.com>
Commit-Queue: Stéphane Marchesin <marcheu@google.com>
Tested-by: Stéphane Marchesin <marcheu@google.com>