support drm_user_handler to handle vendor events

support drm_user_handler to handle vendor events

Add drm_user_hander to handle vendor drm events like ipp events.

Note: From tizen, this is used from libtdm vendor layer.

Change-Id: I23149a08d9f9f66f9929d95dcfe6a96c1c897404
Signed-off-by: Boram Park <boram1288.park@samsung.com>
check if user_handler_list inited

Change-Id: I95abd7e8cc8d0485a7cc06dca54bfa46775eb8aa
Signed-off-by: Boram Park <boram1288.park@samsung.com>
[inki.dae: add omitted patch from tizen libdrm-2.4.75]
Signed-off-by: Inki Dae <inki.dae@samsung.com>
[sw0312.kim: squash user_handler patches]
Signed-off-by: Seung-Woo Kim <sw0312.kim>