drm/i915/kvmgt: fix an error code in gvt_dma_map_page()

The dma_mapping_error() returns true on error but we want to return
-ENOMEM here.

Fixes: 79e542f5 ("drm/i915/kvmgt: Support setting dma map for huge pages")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>