drm/i915: Rename get stolen functions for LP platforms chv+

gen8 is used for both Broadwell and Cherryview but this
function here is only Cherryview and all next atom LP platforms.
So let's rename it to avoid confusion as suggested by Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.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/1482096988-400-2-git-send-email-rodrigo.vivi@intel.com