drm/i915: Consolidate some open coded mmio rmw

Replace some gen6/7 open coded rmw with intel_uncore_rmw.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-14-tvrtko.ursulin@linux.intel.com