drm/i915/guc: Default to non-verbose GuC logging

Now that we've decoupled logging from relay, GuC log level is only
controlling the GuC behavior - there shouldn't be any impact on i915
behaviour. We're only going to see a single extra interrupt when log
will get half full.
That, and the fact that we're seeing igt/gem_exec_nop/basic-series
failing with non-verbose logging being disabled.

v2: Bring back the "auto" guc_log_level, now that we fixed the log

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319095348.9716-11-michal.winiarski@intel.com