drm/i915/guc: Reduce verbosity on log overflows

If the user is clearing the log buffer too slowly, we overflow. As this
is an expected condition, and the driver tries to handle it, reduce the
error message down to a notice.

Michal mentioned that another cause would be incorrect reset handling,
so we don't want to lose the notification entirely.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110817


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617100917.13110-1-chris@chris-wilson.co.uk