drm/i915/guc: Allow user to control default GuC logging

While both naming and actual log enable logic in GuC interface are
confusing, we can simply expose the default log as yet another log
level.
GuC logic aside, from i915 point of view we now have the following GuC
log levels:
	0 Log disabled
	1 Non-verbose log
	2-5 Verbose log

v2: Adjust naming after rebase.
v3: Fixed the log_level logic error introduced on rebase.

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/20180319095348.9716-10-michal.winiarski@intel.com