drm/i915: Move intel_(pre_disable/post_enable)_primary to intel_display.c, and use it there.

They're the same code, so why not?

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>