drm/amdkfd: Fix bug in pipelines initialization

This patch fixes a bug when calling to init_pipeline() interface.
The index that was passed to that function didn't take into account the
first_pipe value, which represents the first pipe index that is under amdkfd's
responsibility.

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>