drm/i915/gvt: Utility for valid command length check

Add utility for valid command length check.

v2: Add F_VAL_CONST flag to identify the value is const
    although LEN maybe variable. (Zhenyu)
v3: unused code removal, flag rename/conflict. (Zhenyu)
v4: redefine F_IP_ADVANCE_CUSTOM and move the check function to
    next patch. (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>