drm/i915: Remove atomic.pre_disable_primary.

This can be derived from the atomic state in pre_plane_update,
which makes it more clear when it's supposed to be called.

Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454514805-10595-3-git-send-email-maarten.lankhorst@linux.intel.com