drm/i915/sdvo: Grab mode_config.mutex during LVDS init to avoid WARNs

drm_mode_probed_add() is unhappy about being called w/o
mode_config.mutex. Grab it during LVDS fixed mode setup
to silence the WARNs.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7301


Fixes: aa2b8807 ("drm/i915/sdvo: Fix multi function encoder stuff")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026101134.20865-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>