drm/i915/guc: Flush directly in log unregister

drm/i915/guc: Flush directly in log unregister

Having both guc_flush_logs and guc_log_flush functions is confusing.
While we could just rename things, guc_flush_logs implementation is
quite simple. Let's get rid of it and move its content to unregister.

v2: s/dev_priv/i915 (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/20180319095348.9716-4-michal.winiarski@intel.com