drm: Make __drm_object_property_get_value() static

Make __drm_object_property_get_value() static to fix the following
sparse warning:
drm_mode_object.c:250:5: warning: symbol '__drm_object_property_get_value' was not declared. Should it be static?

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 4a97a3da

 ("drm: Don't update property values for atomic drivers")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901165328.24459-4-ville.syrjala@linux.intel.com
Reviewed-by: Thierry Reding <treding@nvidia.com>