drm/msm: Nuke preclose hooks

They only complete the page flip events to avoid oops when the drm
file closes. The core takes care of that now and we can remove this
code.

Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-8-git-send-email-daniel.vetter@ffwll.ch