drm: make drm_file use keyed wakeups

Some processes, such as systemd, are only polling for EPOLLERR|EPOLLHUP.
As drm_file uses unkeyed wakeups, such a poll receives many spurious
wakeups from uninteresting events.

Use keyed wakeups to allow the wakeup target to more efficiently discard
these uninteresting events.

Signed-off-by: Kenny Levinsen <kl@kl.wtf>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200424145103.3048-1-kl@kl.wtf