drm/i915/gvt: Add GEN9_CLKGATE_DIS_4 to default BXT mmio handler

Host prints lots of untracked MMIO at 0x4653c when creating linux guest.
"gvt: vgpu 2: untracked MMIO 0004653c len 4"

GEN9_CLKGATE_DIS_4 (0x4653c) is accessed by i915 for gmbus clockgating.
However vgpu doesn't support any clockgating powergating operations
on related mmio access trap so need add it to default handler.
GEN9_CLKGATE_DIS_4 is accessed in bxt_gmbus_clock_gating() which only
applies to GEN9_LP so doens't show the warning on other platforms.

The solution is to add it to default handler init_bxt_mmio_info().

Reviewed-by: He, Min <min.he@intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>