drm/i915/gvt: Add macro define for mmio 0x50080 and gvt flip event

Add SKL_FLIP_EVENT to address into intel_gvt_event_type for primary
and sprite0 plane flip event.
Add macro to address REG_50080 offset.

v2:
Add bit operation definition for flip mode. (zhenyu)

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>