drm/i915/guc: Log significant events at the info level

Currently to establish whether GuC firmware has been loaded or
submission enabled (default DRM log level), one has to detect
the absence of the message saying that the load has been skipped
and infer the opposite.

It is better to log the fact GuC firmware has been loaded and/or
submission enabled explicitly to avoid any guesswork when looking
at the logs.

v2:
  * Log message polish. (Chris)
  * Future proof by reporting found firmware version. (Michal)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1486457425-32548-1-git-send-email-tvrtko.ursulin@linux.intel.com