drm: Kill ctx_count from struct drm_device

The only user of ctx_count is the via driver, and we can replace that
use with list_is_singular().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fops.c

@@ -524,7 +524,6 @@ int drm_release(struct inode *inode, struct file *filp)
 
 				list_del(&pos->head);
 				kfree(pos);
-				--dev->ctx_count;
 			}
 		}
 	}