drm/i915: group ADPA #defines together

Splitting them up between pch and gmch variants just makes it harder
to find things. Especially since the hotplug bits are actually valid
on earlier chips, too.

v2: Fixed the comment as pointed out by Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>