drm/i915/gvt: remove the unused sreg

code cleanup. sreg is not used now. remove it for code cleanness.

v3: remove unnecessary array_size in vreg's memory allocation (min he)
v2: do not allocate memory for sreg. (min he)

Reviewed-by: He, Min <min.he@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>