drm/i915: Organize GEN features inheritance.

As Chris noticed the current organization is confusing
and inheritance is not clear.

So, let's split it in GEN<n>_FEATURES <cdn>_PLATFORM
where new GEN inherit features from previous gens and
Platforms only use gen features plus what ever is specific
for that platform and shouldn't be passed on.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171003063652.17248-2-rodrigo.vivi@intel.com