drm/i915/guc: Tidy guc_log_control

We plan to decouple log runtime (mapping + relay) from verbosity control.
Let's tidy the code now to reduce the churn in the following patches.

v2: Tidy macros, keep debug messages, use helper var for enable,
    correct typo (Michał)
    Fix incorrect input validaction (Sagar)

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: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@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/20180308154707.21716-1-michal.winiarski@intel.com