drm/i915: Polish some RMWs

Doing the if-else around RMWs is kinda silly. Just set/clear the
apporiate bits with a single RMW.

Also unify the coding style a bit icl_wa_cursorclkgating() while at it.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231121054324.9988-4-ville.syrjala@linux.intel.com