drm/i915/ipc: use intel_uncore_rmw() to enable/disable

Don't duplicate the rmw function.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/113a17cd18401b0e4c83396575b67aa6efb07346.1662983005.git.jani.nikula@intel.com