drm/i915/gvt: update vreg on inhibit context lri command

Commit cd7e 61b9"init mmio by lri command in vgpu inhibit context"
initializes registers saved/restored in context with its vreg value
through lri command in ring buffer. It relies on vreg got updated
on every guest access. There is a case found that Linux guest uses
lri command in inhibit-ctx to update the register. This patch adds
vreg update on this case.

v2: move mmio_attribute functions to gvt.h (Zhenyu)
v3: use mask_mmio_write in vreg update
v4: refine codes and add more comments (Zhenyu)

Fixes: cd7e61b9("drm/i915/gvt: init mmio by lri command in vgpu inhibit context")
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>