drm/i915: Make skl+ universal plane registers unlocked

Drop the locks around most universal plane register writes.
The lock isn't needed since each plane's register are neatly
contained on their own cachelines.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220210062403.18690-3-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>