drm/i915/icl: Switch to using 12 deep CSB status FIFO

Now when we can support variable csb fifo sizes, disable legacy mode.
By disabling legacy we hope to get better hw testing coverage by
assuming everyone else have switched over.

v2: rebase

References: https://bugs.freedesktop.org/show_bug.cgi?id=110338


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Kelvin Gardiner <kelvin.gardiner@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405204657.12887-2-chris@chris-wilson.co.uk