drm/bridge: analogix_dp: Downgrade "Link Training" messages to dev_dbg

The Analogix DP bridge driver is pretty verbose, and outputs
things like

[  619.414067] rockchip-dp ff970000.edp: Link Training Clock Recovery success
[  619.429233] rockchip-dp ff970000.edp: Link Training success!

each time the display gets unblanked. While it is good to know
that the device is behaving correctly, users already know that
because they can see some video output.

Let's keep these messages for cases where we need to actually
debug the driver (we have dynamic debug to enable them at runtime
if need be), and let's keep the kernel quiet otherwise.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180805172857.2517-1-marc.zyngier@arm.com