drm/i915/guc: Keep GuC log disabled by default

drm/i915/guc: Keep GuC log disabled by default

It looks that GuC log functionality is not fully functional yet and
causes issues when enabled by auto(-1) modparam on debug builds.
For example, but not limited to:

[   30.062893] ======================================================
[   30.062894] WARNING: possible circular locking dependency detected
[   30.062895] 4.15.0-rc8-CI-CI_DRM_3648+ #1 Tainted: G     U
[   30.062896] ------------------------------------------------------
[   30.062897] debugfs_test/1268 is trying to acquire lock:
[   30.062898]  (&dev->struct_mutex){+.+.}, at: [<00000000e4213449>] i915_mutex_lock_interruptible+0x47/0x130 [i915]
[   30.062921]
               but task is already holding lock:
[   30.062921]  (&mm->mmap_sem){++++}, at: [<00000000dd7adc93>] __do_page_fault+0x106/0x560
[   30.062924]
               which lock already depends on the new lock.

References: 0ed87953532652 ("drm/i915/guc: Redefine guc_log_level modparam values")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104693
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104694
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104695
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Saarinen <jani.saarinen@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Marta Lofstedt <marta.lofstedt@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180119124926.29844-1-michal.wajdeczko@intel.com
Reviewed-by: Michal Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>