drm/bridge: stm_lvds: Do not fail atomic_check on disabled connector

drm/bridge: stm_lvds: Do not fail atomic_check on disabled connector

If the connector is disabled, the new connector state has .crtc field
set to NULL and there is nothing more to validate after that point.
The .crtc field being NULL is not an error. Test for .crtc being NULL,
and if it is NULL, exit early with return 0.

This fixes a failure in suspend/resume path, where the connector is
already disabled, but .atomic_check is called, fails, returns -EINVAL
and blocks the suspend entry.

Fixes: aca1cbc1c986 ("drm/stm: lvds: add new STM32 LVDS Display Interface Transmitter driver")
Signed-off-by: Marek Vasut <marex@nabladev.com>
Acked-by: Raphaël Gallais-Pou <raphael.gallais-pou@foss.st.com>
Link: https://patch.msgid.link/20260409024928.344010-1-marex@nabladev.com
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>