drm/atomic: Check for busy planes/connectors before setting the commit

We still want to fail with -EBUSY if a plane or connector is part of
a commit, even if it will be assigned to a new commit.

This closes a small hole left open where we should return -EBUSY.
It's not severe, because wait_for_dependencies and swap_state helpers
still block. But it should return -EBUSY and not stall.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 21a01abb ("drm/atomic: Fix freeing connector/plane state too early by tracking commits, v3.")
Link: https://patchwork.freedesktop.org/patch/msgid/20171016132928.6498-2-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>