drm/amdgpu: fix incorrect use of amdgpu_irq_add_id in si_dma.c

Adding a second irq source because of a different src_id is actually a
bug.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>