drm/i915: Use single_enabled_crtc() in i9xx_update_wm()

Replace the ad-hoc single_enabled_crtc() thing in i9xx_update_wm()
with the real thing, just like we do in the other legacy wm functions.
We can also nuke the extra 'enabled' variable.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211209144311.3221-3-ville.syrjala@linux.intel.com


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