drm/i915: Update description of i915.enable_guc modparam

Commit f774f096 ("drm/i915/guc: Turn on GuC/HuC auto mode") changed
the default from 0 to -1 but forgot to update the description.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: f774f096 ("drm/i915/guc: Turn on GuC/HuC auto mode")
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717104418.23809-1-tvrtko.ursulin@linux.intel.com