drm/i915/gvt: add some new MMIOs to cmd_access white list

Guest is now acces some MMIOs (0x215c, RING_INSTPM) through command which
is not originally in gvt's white list. This cause huge error log printed in
gvt.
This patch addes these MMIOs to the white list.

V2. change the commit message content.
V3. remove duplicate defination of 0x20c0.
V4. refine code style.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>