drm/i915/gvt: Enable async flip on plane surface mmio writes

According to Intel GFX PRM on 01.org, plane surface address can be updated
synchronously or asynchronously. Synchronous flip will hold plane surface
address update to start of next vsync, which is current implementation.
Asynchronous flip will update the address as soon as possible. Without
async flip, some 3D application could not reach better performance and
the maximum performance is no higher than vsync frequency.

The patch enables the async flip on plane surface address mmio update,
and increment flip count correctly.

With async flip enabled, some 3D applications have significant performance
improvement. i.e. 3DMark Ice Storm has a 300%~400% increment on score.

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>