drm/i915: Avoid stalling on pending flips for legacy cursor updates

drm/i915: Avoid stalling on pending flips for legacy cursor updates

The legacy cursor ioctl expects to be asynchronous with respect to other
screen updates, in particular page flips. As X updates the cursor from a
signal context, if the cursor blocks then it will stall both the input
and output chains causing bad stuttering and horrible UX.

Reported-and-tested-by: Rafael Ristovski <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94980
Fixes: 5008e874edd34 ("drm/i915: Make wait_for_flips interruptible.")
Suggested-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: [email protected]
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Acked-by: Daniel Vetter <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>