drm/i915: Extract intel_sanitize_fifo_underrun_reporting()

Pull the underrun status sanitation into its own helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220615174851.20658-1-ville.syrjala@linux.intel.com


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