amdkfd: properly free gang_ctx_bo when failed to init user queue

The destructor of a gtt bo is declared as
void amdgpu_amdkfd_free_gtt_mem(struct amdgpu_device *adev, void **mem_obj);
Which takes void** as the second parameter.

GCC allows passing void* to the function because void* can be implicitly
casted to any other types, so it can pass compiling.

However, passing this void* parameter into the function's
execution process(which expects void** and dereferencing void**)
will result in errors.

Signed-off-by: Zhu Lingshan <lingshan.zhu@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Fixes: fb910658 ("drm/amdkfd: Refactor queue wptr_bo GART mapping")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>