drm/amdkfd: Fix an inappropriate error handling in allloc memory of gpu

We should unreference a gem object instead of an amdgpu bo here.

Fixes: fd9a9f88

 ("drm/amdgpu: Use GEM obj reference for KFD BOs")

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>