drm/amdkfd: drop IOMMUv2 support

Now that we use the dGPU path for all APUs, drop the
IOMMUv2 support.

v2: drop the now unused queue manager functions for gfx7/8 APUs

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>