drm/i915/gvt: Allow F_CMD_ACCESS on mmio 0x21f0

The newly updated guest driver could program 0x21f0 via lri and srm,
without F_CMD_ACCESS flag cmd parser will stop parser reset cmd.
0x21f0 applies to BDW, SKL, KBL, BXT and CFL.

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