drm: simplify master cleanup

In drm_master_destroy() we _free_ the master object. There is no reason to
hold any locks while dropping its static members, nor do we have to reset
it to 0.

Furthermore, kfree() already does NULL checks, so call it directly on
master->unique and drop the redundant reset-code.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>