drm/amdgpu/powerplay: use irq source defines for smu7 sources

Use the newly added irq source defines rather than magic numbers
for smu7 thermal interrupts.

Rewiewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Rex Zhu <rezhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>