drm/i915/guc: Don't try to enable GuC logging when we're not using GuC

When changing the default values for guc_log_level, we accidentally left
the log enabled on non-guc platforms. Let's fix that.

v2: Define the levels used and remove (now obsolete) comments (Chris)
v3: Use "IS" rather than "TO" for booleans (Chris)

Fixes: 9605d1ce ("drm/i915/guc: Default to non-verbose GuC logging")
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320115517.20423-1-michal.winiarski@intel.com