drm/tilcdc: remove submodule destroy calls

The TI tilcdc driver is designed with a notion of submodules. Currently,
at unload time, these submodules are iterated and destroyed.

Now that the tilcdc remove order is fixed, this can be handled perfectly
by the kernel using the device infrastructure, since each submodule
is a kernel driver itself, and they are only destroy()'ed at unload
time. Therefore we move the destroy() functionality to each submodule's
remove().

Also, remove some checks in the unloading process since the new code
guarantees the resources are allocated and need a release.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Tested-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>