Revert "drm/i915: Update description of i915.enable_guc modparam"

This reverts commit 0629d4da.

If GuC firmware is not present on the filesystem driver crashes the
machine on boot.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Fixes: 0629d4da ("drm/i915: Update description of i915.enable_guc modparam")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
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
Link: https://patchwork.freedesktop.org/patch/msgid/20190719094845.6242-3-tvrtko.ursulin@linux.intel.com