drm/i915: Remove platforms in the preliminary_hw_support description

We already managed to get it out of sync (Haswell has been promoted out
of this option), so let's remove all mentions to platforms.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>