drm/i915/mst: Fix .mode_valid_ctx() return values

drm/i915/mst: Fix .mode_valid_ctx() return values

.mode_valid_ctx() returns an errno, not the mode status. Fix
the code to do the right thing.

Cc: stable@vger.kernel.org
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: d51f25eb479a ("drm/i915: Add DSC support to MST path")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127145028.4899-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>