drm: Add missing mutex_destroy in drm_dev_init/release

Add 3 missing mutex_destroy to drm_dev_init teardown and
drm_dev_release.

v2:
- Also include drm_dev_release

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478785835-4142-1-git-send-email-joonas.lahtinen@linux.intel.com
Link: http://patchwork.freedesktop.org/patch/msgid/1478785835-4142-1-git-send-email-joonas.lahtinen@linux.intel.com