drm/i915: Finish any pending operations on the framebuffer before disabling

Similar to the case where we are changing from one framebuffer to
another, we need to be sure that there are no pending WAIT_FOR_EVENTs on
the pipe for the current framebuffer before switching. If we disable the
pipe, and then try to execute a WAIT_FOR_EVENT it will block
indefinitely and cause a GPU hang.

We attempted to fix this in commit 85345517
(drm/i915: Retire any pending operations on the old scanout when switching)
for the case of mode switching, but this leaves the condition where we
are switching off the pipe vulnerable.

There still remains the race condition were a display may be unplugged,
switched off by the core, a uevent sent to notify the DDX and the DDX
may issue a WAIT_FOR_EVENT before it processes the uevent. This window
does not exist if the pipe is only switched off in response to the
uevent. Time to make sure that is so...

Reported-by: Francis Leblanc <Francis.Leblanc-Lebeau@verint.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36515
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45413


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: fixup spelling in comment, noticed by Eugeni.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>