drm/i915/gvt: let force_to_nonpriv cmd handler only valid for LRI cmd

the cmd_reg_handler() is called by cmds LRM, PIPE_CTRL, SRM...
for LRM, SRM, we cannot get write data in a simple way.
On other side,  the force_to_nonpriv reigsters will only be written in LRI
in current drivers. so we don't want to bother the handler to handle those
memory access cmds, just leave a print message here.

Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>