drm/amdkfd: remove an unnecessary amdgpu_bo_ref

No need to reference the BO here, dmabuf framework will handle that.

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>