drm/imx: Unconfuse preclose logic

So this one is special, since it tries to prevent races when userspace
crashes simply by disabling the vblank machinery. Well except that imx
always has vblanks enabled, and the disable_vblank hook actually just
tries to cancel a pending pageflip. Without any locking whatsoever. Of
course this is wrong, since it'll result in the hw not actually
displaying what drm thinks is the current frontbuffer.

Well since the core takes care of the disappearing DRM fd now. So we
can nuke all this confused code without ill side-effects.

Someone else needs to audit the locking for ->newfb and
->page_flip_event and fix it up. Common approach is to reuse
dev->event_lock for this.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-7-git-send-email-daniel.vetter@ffwll.ch