drm: Add functions to setup/tear down drm_events.

An attempt at not spreading out the file_priv->event_space stuff out
quite so far and wide.  And I think fixes something in ipp_get_event()
that is broken (or if they are doing something more weird/subtle, then
breaks it in a fun way).

Based upon a patch from Rob Clark, rebased and polished.

v2: Spelling fixes (Alex).

Cc: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Rob Clark <robdclark@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452548477-15905-3-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>