drm/i915/gvt: add cmd_access to GEN7_HALF_SLICE_CHICKEN1

Linux guest is using this MMIO in lri command. Add cmd_access flag
for this mmio in gvt to avoid error log.

v2: change the mmio address to its macro name

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>