drm/i915/guc: Log runtime should consist of both mapping and relay

Currently, we're treating relay and mapping of GuC log as a separate
concepts. We're also using inconsistent locking, sometimes using
relay_lock, sometimes using struct mutex.
Let's correct that. Anything touching the runtime is now serialized
using runtime.lock, while we're still using struct mutex as inner lock
for mapping.
We're still racy in setting the log level - but we'll take care of that
in the following patches.

v2: Tidy locking (Sagar)
v3: Remove obsoleted comment (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: 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/20180319095348.9716-2-michal.winiarski@intel.com