drm/vc4: Drop debug print at boot with DPI enabled.

Unlike the other encoders in the driver, I've also dropped the debug
dump function.  There's only really one register to this device, and
we have the debugfs reg entry still.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170208204701.29013-1-eric@anholt.net