IH is the interrupt handler block.

v1: add initial ih support (Ray)
v2: add dummy prescreen iv function for navi10 (Hawking)
v3: squash in additional updates (Alex)
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>