drm/crc: Actually allow to change the crc source

Oops.

Fixes: 9edbf1fa ("drm: Add API for capturing frame CRCs")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821203835.18314-1-daniel.vetter@ffwll.ch