If the bridge is attached with the DRM_BRIDGE_ATTACH_NO_CONNECTOR flag set,
this driver won't initialize a connector and hence display mode won't be
validated in drm_connector_helper_funcs::mode_valid().  So, move the mode
validation from drm_connector_helper_funcs::mode_valid() to
drm_bridge_funcs::mode_valid(), because the mode validation is always done
for the bridge.

Fixes: 30e2ae94 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240813091637.1054586-1-victor.liu@nxp.com