drm/i915/gvt: Add valid length check for MI variable commands

Add valid length check for the commands with variable length.

v2: remove the macro definition. (Zhenyu)
v3: refine the LRI command. (Zhenyu)

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Gao, Fred <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>