drm/amdgpu: fix metadata_size for ubo ioctl queries

Although the kfd_ioctl_get_dmabuf_info() still fail it will indicate
the caller right metadat_size useful for the same kfd ioctl next time.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>