drm/bridge: cdns-dsi: Move DSI mode check to _atomic_check()

drm/bridge: cdns-dsi: Move DSI mode check to _atomic_check()

At present, the DSI mode configuration check happens during the
_atomic_enable() phase, which is not really the best place for this.
Moreover, if the mode is not valid, the driver gives a warning and
continues the hardware configuration.

Move the DSI mode configuration check to _atomic_check() instead, which
can properly report back any invalid mode, before the _enable phase even
begins.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
Signed-off-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
Link: https://lore.kernel.org/r/20250329113925.68204-10-aradhya.bhatia@linux.dev
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>