drm/i915: Put future HW and their uAPIs under STAGING & BROKEN

We would like some freedom to break the user API/ABI for future HW but
yet still expose the driver for upstream development on that HW.
Currently, we have the i915.force_probe module parameter to avoid binding
to HW while the driver is under development, but that is still a little
too soft with respect to the stringent no-regression rules if we also
plan to be redesigning the uAPI to go along with the new HW.

To allow the uAPI to be changed during development, only expose that API
and in development HW under STAGING (and BROKEN). Hopefully, making it
explicit that such interfaces to that HW are under development and not
to be blindly enabled by distributions.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191027154314.11139-1-chris@chris-wilson.co.uk