drm/sun4i: backend: Move line stride setup to buffer setup function

Setup the line stride in the buffer setup function, since it's tied to the
buffer itself, and is not needed when we do not set the buffer in the
backend.

This is for example the case when using the frontend and then routing its
output to the backend.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cbec84125bc0d5a6cf1d856b8291fbf77b138881.1516613040.git-series.maxime.ripard@free-electrons.com