drm/i915: Make pre-skl sprite plane registers unlocked

Drop the locks around sprite 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/20220210062403.18690-6-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>