drm/i915/csr: Allow matching unknown HW steppings with generic firmware

If the firmware is generic and has a run-anywhere mode, enable it rather
than completely failing on unknown HW revisions.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457352357-8433-1-git-send-email-chris@chris-wilson.co.uk