drivers: Move iommu/ before gpu/ in Makefile

AMD GPU devices are dependent on AMD IOMMU controller functionality to allow
the GPU to access a process's virtual memory address space, without the need
for pinning the memory.

This patch changes the order in the drivers makefile, so iommu/ subsystem is
linked before gpu/ subsystem. That way, if the gpu and iommu drivers are
compiled inside the kernel image (not as modules), the correct order of device
loading is still maintained (iommu module is loaded before gpu module).

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>