drm: arcpgu: Fix module unload

At module unload we are expecting a struct drm_device but at
probing we are not setting it right. Fix this and correct the
arcpgu module unload.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Fixes: 0c4250e7b15e ("drm: Add support of ARC PGU display controller")
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>