drm/i915: Simplify gem stolen initialization.

Let's take usage of IS_LP to simplify the gem stolen
initialization as suggest by Tvrtko.

Also assume that all new LP platforms follows the chv+
and others bdw+.

v2: Remove the wrong commit message about bxt and glk. (Ander)

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
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/1482174347-24911-1-git-send-email-rodrigo.vivi@intel.com