drm/vblank: remove outdated and noisy output

The R-Car DU driver calls drm_vblank_init via some helper functions in
probe(). From what I checked, most drivers do this as well. I have a
config now where DU always stays in deferred_probe state because of a
missing dependency. This means that every time I rebind another driver
like MMC, the vblank init message is displayed again when the DU driver
is retried. Because the message doesn't really carry a useful
information, I suggest to simply drop it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513201016.23047-1-wsa+renesas@sang-engineering.com