drm/amdgpu/discovery: Need to free discovery memory

When unloading driver, need to free discovery memory.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>