drm/atomic: Change drm_atomic_helper_swap_state to return an error.

We want to change swap_state to wait indefinitely, but to do this
swap_state should wait interruptibly. This requires propagating
the error to each driver.

Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170711143314.2148-3-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[mlankhorst: Fix typos in swap_state documentation (seanpaul)]
Reviewed-by: Sean Paul <seanpaul@chromium.org&gt;]>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>