drm/i915/gvt: Change the return type during command scan

Generally, there are 3 types of errors during command scan: a) some
commands might be unknown with EBADRQC;  b) some cmd access invalid
address with EFAULT; c) some unexpected force nonpriv cmd with EPERM.
later the healthy state can be judged through the return error.

v2:
- remove some internal i915 errors rating.  (Zhenyu)

v3:
- the healthy state is judged through the internal defined return
  error. (Zhenyu)
- force non priv cmd error can be ignored. (Kevin)

v4:
- reuse standard defined errno instead of recreate, e.g EBADRQC for
  unknown cmd, EFAULT for invalid address, EPERM for nonpriv. (Zhenyu)

v5:
- remove some irrelevant code for the patch.
- fix typo of vgpu_is_vm_unhealthy. (Zhenyu)

v6:
- move the healthy check and failsafe code into another patch. (Zhenyu)

v7:
- polish title and commit message. (Zhenyu)

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