drm/i915/gvt: Update force-to-nonpriv register whitelist

Host prints below warning message when guest running some application:
"gvt: vgpu(1) Invalid FORCE_NONPRIV write 2754 at 24f0".
"gvt: vgpu(1) Invalid FORCE_NONPRIV write 28a0 at 24f0".

Registers 0x2754 and 0x28a0 are required by guest so add to whitelist.

Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>