drm/amdkfd: Fix NULL pointer dereference

Check that adev->gfx.ras is valid before using it.

Fixes: f385fc0d ("drm/amdgpu: add UTCL2 RAS poison query for Aldebaran (v2)")
CC: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>