drm/atomic-helpers: Properly avoid full modeset dance
drm/atomic-helpers: Properly avoid full modeset dance

Legacy setCrtc has a nice fastpath for just updating the frontbuffer
when the output routing doesn't change. Which I of course tried to
keep working, except that I fumbled the job: The helpers correctly
compute ->mode_changed, CRTC updates get correctly skipped but
connector functions are called unconditionally.

Fix this.

v2: For the disable sided connector->state->crtc might be NULL.
Instead look at the old_connector_state->crtc, but still use the new
crtc state for that old crtc. Reported by Thierry.

Reported-and-Tested-by: Gustavo Padovan <029a69b25713d0799b81c0a39685b4c040fa1425@collabora.co.uk>
Reviewed-by: Gustavo Padovan <029a69b25713d0799b81c0a39685b4c040fa1425@collabora.co.uk> (v1)
Cc: Thierry Reding <b75a4d02013b0e68ae2c6bb0dcfd4b7bec282dc9@gmail.com>
Cc: Gustavo Padovan <029a69b25713d0799b81c0a39685b4c040fa1425@collabora.co.uk> (v1)
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@intel.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs