drm/omap: Move DISPC timing checks to CRTC .mode_valid() operation

The DISPC timings checks relate to the CRTC, but they're performed in
the encoder and connector .atomic_check() and .mode_valid() operations.
Move them to the CRTC .mode_valid() operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>