drm/i915/guc: Change platform default GuC mode

Today our most desired GuC configuration is to only enable HuC
if it is available (as we need authenticated HuC firmware to enable
all media codecs on the hardware) and we really don't care about
having GuC submission enabled.

Change platform default GuC mode to match our goal, but note that
we still don't change default modparam value (GuC/HuC disabled).

v2: add why HuC is so important (Joonas)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Spotswood <john.a.spotswood@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Tony Ye <tony.ye@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Jeff Mcgee <jeff.mcgee@intel.com>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Acked-by: Tony Ye <tony.ye@intel.com>
Reviewed-by: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527183613.17076-2-michal.wajdeczko@intel.com