drm/legacy: Convert setplane ioctl locking to interruptible.

Pass DRM_MODESET_ACQUIRE_INTERRUPTIBLE to acquire_init, and handle
drm_modeset_backoff which can now fail by returning the error.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912133749.6532-5-maarten.lankhorst@linux.intel.com
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>