drm/i915: Fix port sync code to work with >2 pipes

Don't assume there is just one port sync slave. We might have several.

v2: Fix unitialized new_crtc_state usage (José)
    Fix clearing of modeset_pipes for slaves
    s/oldnew/new/

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313164831.5980-11-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>