drm: Deduplicate driver initialization message

Several DRM drivers print the same initialization message right after
drm_dev_register, so move that to common code.  The exception is i915,
which uses its own register handle, so let it keep its own message.

Notice that this was tested only with Exynos, but looks simple enough
for the other drivers.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161228143216.26821-2-krisman@collabora.co.uk