drm/armada: move mode set vblank handling and disable/enable

Move the mode set vblank handling and controller enable/disable to the
prepare() and commit() callbacks.  This will be needed when we move to
mode_set_nofb() as we should not enable the controller without the
plane coordinates and location having been properly updated.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>