drm/amdgpu: set sdma irq src num according to sdma instances

Otherwise, it will cause driver access non-existing sdma registers
in gpu reset code path

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>