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: NHuang Rui <ray.huang@amd.com>
Signed-off-by: NHawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>