drm/i915/gvt: add write handler for mmio mbctl

Guest will write mmio mbctl which need a special handler in gvt to
clear the bit 4 to inidcate the write operation success.

V2: use bit definition macro to make code readable.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>