drm/i915/gvt: correct mask setting for CSFE_CHICKEN1

CSFE_CHICKEN1(0x20d4) needs access with mask. This is caught in AcrnGT
conformance check test:

[drm:intel_gvt_vgpu_conformance_check]
	*ERROR* gvt: vgpu1 unconformance mmio 0x20d4:0x40004,0x4

Signed-off-by: Xinyun Liu <xinyun.liu@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>