drm/amdkfd: check for null dev to avoid a null pointer dereference

The call to kfd_device_by_id can potentially return null, so check that
dev is null and return with -EINVAL to avoid a null pointer dereference.

Detected by CoverityScan CID#1454629 ("Dereference null return value")

Fixes: 5d71dbc3

 ("drm/amdkfd: Implement image tiling mode support v2")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>