drm/amd: Add interrupt source definitions for SOC15 v3.

Stop using 'magic numbers' when registering interrupt sources.

v2: Switch to kernel style comments.

v3:
Rebase.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>