drm/i915: intel_update_fbc() requires struct_mutex, so no longer atomic

As we need to manipulate our device structure and allocate queue a task,
it is no longer a simple atomic operation and cannot be performed along
the atomic modeset paths. Instead make sure that we disable FBC (which
must be therefore kept as a set of simple register writes) when
performing the atomic modeset and leave the heavy-weight
intel_update_fbc() for the normal modeset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>