drm/i915: Make plane fb tracking work correctly, v2.

atomic->disabled_planes is a hack that had to exist because
prepare_fb was only called when a new fb was set. This messed
up fb tracking in some circumstances like aborts from
interruptible waits. As a result interruptible waiting in
prepare_plane_fb was forbidden, but other errors could still
cause frontbuffer tracking to be messed up.

Now that prepare_fb is always called, this hack is no longer
required and prepare_fb may fail without consequences.

Changes since v1:
- Clean up a few fb tracking warnings by changing plane->fb to
  plane->state->fb.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>