drm: Constify drm_mode_config_funcs pointer

The DRM mode config functions structure declared by drivers and pointed
to by the drm_mode_config funcs field is never modified. Make it a const
pointer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Rob Clark <rob.clark@linaro.org>
Reviwed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>