drm/i915/gvt: Fix check error on fence mmio handler

Fix below error with minor code refactor.

CHECK   drivers/gpu/drm/i915//gvt/handlers.c
drivers/gpu/drm/i915//gvt/handlers.c:203 sanitize_fence_mmio_access() error: 'vgpu' dereferencing possible ERR_PTR()

Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>