drm/i915: Demote i915_gem_open() debugging from DRIVER to USER

We use DRM_DEBUG() when reporting on user actions, to try and keep
intentional errors out of the CI dmesg. Demote the debug from
i915_gem_open() similarly so that it is only apparent with drm.debug & 1
like its brethren.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161109104507.21228-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>