drm/amdkfd: Fix handling of return code of dma_buf_get

On errors, dma_buf_get returns a negative error code, rather than NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>