drm/i915/gvt: use kmap instead of kmap_atomic around guest memory access

kmap_atomic doesn't allow sleep until unmapped. However,
it's necessary to allow sleep during reading/writing guest
memory, so use kmap instead.

Signed-off-by: Bing Niu <bing.niu@intel.com>
Signed-off-by: Xiaoguang Chen <xiaoguang.chen@intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>