drm/amdgpu: Remove hard-coded assumptions about compute pipes

Remove hard-coded assumption that the first compute pipe is
reserved for amdgpu. Pipe 0 actually means pipe 0 now.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>