drm/i915/gvt: update RING_START reg of vGPU when the context is submitted to i915

The guest may use this register to identify the running state of one
context. Emulate it as the value in context image as if the context runs
on the GPU hardware.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>