drm/i915/gvt: return error on cmd access

If a register is not cmd accessible, should not just print error
message. Return error here so as not to deliver this cmd.

v2: return -EBADRQC to align with return value elsewhere. (kevin tian)

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