drm: xilinx: drv: unref drm device in error path

drm: xilinx: drv: unref drm device in error path

The drm device should be unreferenced properly in the error path.
Otherwise, there's a leak, and the drm device id (card) increments.

Reported-by: Madhurkiran Harikrishnan <madhurki@xilinx.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>