drm/i915: Expand is_lp backwards to gen8_lp and gen7_lp.

Valleyview/Baytrail (gen7_lp) and Cherryview/Braswell (gen8_lp)
are both Atom platforms like Broxton/Apollolake and Geminilake.

So let's expand this is_lp back to these platforms and
create the IS_LP(dev_priv) so we can start simplifying a bit
our if/else for platform lists.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482096988-400-1-git-send-email-rodrigo.vivi@intel.com