drm: Reduce verbosity level for drm_core_init() debug message

Currently at the end of drm_core_init() we print

[    0.735185] [drm] Initialized

which does not provide any user information and is only a breadcrumb for
developers, so reduce it from info to debug.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161229133729.32673-1-chris@chris-wilson.co.uk