drm: remove legacy drm_arm_vblank_event()

We don't have any user of this function anymore, let's remove it.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465308482-15104-3-git-send-email-gustavo@padovan.org