drm/i915/gvt: refine pcode write emulation

In GVT-g we always emulate as pcode read/write success and ready for access
anytime, since we don't touch real physical registers here.

Add 'SKL_PCODE_CDCLK_CONTROL' write emulation, without it will cause
skl_set_cdclk fail in guest.

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