drm: Remove drm_pending_event->pid

We might as well dump the drm_file pointer, that's about as useful
a cookie as the pid. Noticed while typing docs for drm_file and friends.

Since the only consumer of this is the tracepoints I think we can safely
change this - those tracepoints should not be uapi relevant at all. It
all goes back to

commit b9c2c9ae
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Jul 1 16:48:09 2010 -0700

    drm: add per-event vblank event trace points

which doesn't give a special justification for using pid over a pointer.

Also note that the nouveau code setting it is entirely pointless:
Since this isn't a vblank event, it will never hit the vblank
tracepoints.

Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-11-daniel.vetter@ffwll.ch