drm/i915: Define IS_BROXTON properly.

Kabylake will also be defined as gen9 and !is_skylake.
So we need start by creating a proper Broxton
definition, otherwise we will break broxton with the
introduction of Kabylake.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445966099-1640-2-git-send-email-rodrigo.vivi@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>