drm/amdkfd: Use IH context ID for signal lookup

This speeds up signal lookup when the IH ring entry includes a
valid context ID or partial context ID. Only if the context ID is
found to be invalid, fall back to an exhaustive search of all
signaled events.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>