drm: Add DRM-managed mutex_init()

Add drmm_mutex_init(), a helper that provides managed mutex cleanup. The
mutex will be destroyed with the final reference of the DRM device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220502142514.2174-2-tzimmermann@suse.de