drm/fsl-dcu: drop unused drm_crtc_index()

The result of drm_crtc_index() is unused. Remove the call and
the unused variable.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926204150.29829-1-stefan@agner.ch