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 <[e-mail]>
Fixes: 0c4250e7b15e ("drm: Add support of ARC PGU display controller")
Cc: Carlos Palminha <[e-mail]>
Cc: Alexey Brodkin <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Signed-off-by: Alexey Brodkin <[e-mail]>

drivers/gpu/drm/arc/arcpgu_drv.c

···
 		return -ENODEV;
 	}
 
-	platform_set_drvdata(pdev, arcpgu);
+	platform_set_drvdata(pdev, drm);
 	return 0;
 }