drm/i915: Unify skylake plane disable

Don't handle skylake primary plane separately as it is similar
plane as the others.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1508270891-22186-5-git-send-email-juhapekka.heikkila@gmail.com