drm/sun4i: backend: Clarify sun4i_backend_layer_enable debug message

sun4i_backend_layer_enable can be called to enable or disable a layer.
However the debug message always says "Enable", which is confusing.

This patch makes the debug message vary according to the enable state.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>