drm/amdkfd: update user space last_event_age

Update user space last_event_age when event age is enabled.
It is only for KFD_EVENT_TYPE_SIGNAL which is checked by user space.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>