drm/amd/powerplay: move amdgpu_irq_src to the smu structure allocation

Rather than allocating it dynamically at runtime considering it is only
several bytes in size.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>