drm/i915/gvt: Enable synchronous flip on handling MI_DISPLAY_FLIP

According to Intel GFX PRM on 01.org, the MI_DISPLAY_FLIP command can
either request display plane flip synchronously or asynchronously.
In synchronous flip, flip will be hold until next vsync, which
is not implemented yet in GVT. In asynchronous flip, flip will happen
immediately, which is current implementation.

The patch enables the sync flip on handling MI_DISPLAY_FLIP,
and increment flip count correctly by only increment on primary plane.

v2:
Use 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>