drm/i915: add ValleyView driver structs and IS_VALLEYVIEW macro

For use by the rest of the ValleyView code.

v2: fix desktop variant to not set is_mobile (Ben)

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>