drm/atomic: Extract needs_modeset function

We use the same check already in the atomic core, so might as well
make this official. And it's also reused in e.g. i915.

Motivated by Maarten's idea to extract a connector_changed state out
of mode_changed.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-By: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>