drm/amdgpu: introduce new idh_request/event enum

new idh_request and ihd_event to prepare for the
new handshake protocol implementation later

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>