drm: make buffer management work without DRM_MASTER

DRM users should be able to create/destroy/manage dumb- and frame-buffers
without DRM_MASTER. These ioctls do not affect modesetting so there is no
reason to protect them by drm-master. Particularly, destroying buffers
should always be possible as a client has only access to buffers that they
created. Hence, there is no reason to prevent a client from destroying the
buffers, considering a simple close() would destroy them, anyway.

Furthermore, a display-server currently cannot shutdown correctly if it
does not have DRM_MASTER. If some other display-server becomes active (or
the kernel console), then the background display-server is unable to
destroy its buffers.
Under special curcumstances (like monitor reconfiguration) this might even
happen during runtime.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>