drm/i915: Unify GT* and GT3 definitions

This patch clean up a bit the platform definition block in
a way to avoid duplications and to let clear that GT3 for
the current platform only have the extra Media engine (BSD2).

v2: Kabylake IS_KABYLAKE as Anusha noticed.

v3: Avoid EXTRA_ENGINE_MASK and list rings out on GT3 to
    make it more clear.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496765166-7068-1-git-send-email-rodrigo.vivi@intel.com