drm/amdgpu: add missed return value set for error case

Return value should be set when going to error handle tag
for error case, this can avoid potential invalid array
access by upper caller.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>