drm/i915/gvt: Fix kmem_cache_create() name

According to kmem_cache_sanity_check(), spaces are not allowed in the
name of a cache and results in a kernel oops with CONFIG_DEBUG_VM.
Convert to underscores.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>