drm/amdgpu/swsmu: add interrupt work function

So we can schedule work from interrupts.  This might include
long tasks or things that could sleep.

Fixes: e1188aac ("drm/amdgpu/smu11: add support for SMU AC/DC interrupts")
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>