Passing in an invalid DRM overlay id with the --drm-overlay option would
cause drmplane to be freed twice: once in the for-loop and once at the
error-handler label fail.
Solve by setting drmpanel to NULL after freeing it.
Also the 'return false' statement after the error handler label should
probably be 'return NULL', given that the return type of
drm_atomic_create_context returns a pointer.
This commit allows to use the AV_PIX_FMT_DRM_PRIME newly introduced
format in ffmpeg that allows decoders to provide an AVDRMFrameDescriptor
struct.
That struct holds dmabuf fds and information allowing zerocopy rendering
using KMS / DRM Atomic.
This has been tested on RockChip ROCK64 device.