drm/i915/guc: Split relay control and GuC log level

Those two concepts are really separate. Since GuC is writing data into
its own buffer and we even provide a way for userspace to read directly
from it using i915_guc_log_dump debugfs, there's no real reason to tie
log level with relay creation.
Let's create a separate debugfs, giving userspace a way to create a
relay on demand, when it wants to read a continuous log rather than a
snapshot.

v2: Don't touch guc_log_level on relay creation error, adjust locking
    after rebase, s/dev_priv/i915, pass guc to file->private_data (Sagar)
    Use struct_mutex rather than runtime.lock for set_log_level
v3: Tidy ordering of definitions (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/20180319095348.9716-5-michal.winiarski@intel.com