drm/tilcdc: Remove obsolete crtc_mode_valid() hack

Earlier there were no mode_valid() helper for crtc and tilcdc had a
hack to over come this limitation. But now the mode_valid() helper is
there (has been since v4.13), so it is about time to get rid of that
hack.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ <5c4dcb5b1e7975bd2b7ca86f7addf219cd0f9a06.1564750248.git.jsarha@ti.com