drm: Push struct_mutex into ->master_destroy

Only two drivers implement this hook. vmwgfx (which doesn't need it
really) and legacy radeon (which since v1 has been nuked, yay).

v1: Rebase over radeon ums removal.

Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-6-git-send-email-daniel.vetter@ffwll.ch