drm/i915/guc: Merge log relay file and channel creation

We have all the information we need at relay_open call time.
Since there's no reason to split the process into relay_open and
relay_late_setup_files, let's remove the extra code.

v2: Remove obsoleted comments (Sagar)
v3: There was one obsolete comment left (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/20180319095348.9716-3-michal.winiarski@intel.com