drm/i915/gvt: Handle GEN9_WM_CHICKEN3 with F_CMD_ACCESS.

Recent patch introduce strict check on scanning cmd:
Commit 8d458ea0 ("drm/i915/gvt: return error on cmd access")

Before 8d458ea0, if cmd_reg_handler() checks that a cmd access a mmio
that not marked as F_CMD_ACCESS, it simply returns 0 and log an error.
Now it will return -EBADRQC which will cause the workload fail to submit.

On BXT, i915 applies WaClearHIZ_WM_CHICKEN3 which will program
GEN9_WM_CHICKEN3 by LRI when init wa ctx. If it has no F_CMD_ACCESS flag,
vgpu will fail to start. Also add F_MODE_MASK since it's mode mask reg.

v2: Refresh commit message to elaborate issue symptom in detail.
v3: Make SKL_PLUS share same handling since GEN9_WM_CHICKEN3 should be
    F_CMD_ACCESS from HW aspect. (yan, zhenyu)

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