drm/i915/gvt: cleanup GFP flags

In gvt, almost all memory allocations are in sleepable contexts. It's
fault-prone to use GFP_ATOMIC everywhere. Replace it with GFP_KERNEL
wherever possible.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>