drm/i915/gvt: add write vreg in MMIO DMA_CTRL handler

Missing write_vreg in DMA_CTRL write handler would make obsolete
value return when read vreg.

v2: get data from vreg after updating it.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>