drm/sun4i: Fix uninitialized variables in vi layer

min_scale and max_scale in sun8i_vi_layer_atomic_check() can be used
without initialization.

Fix that.

Fixes: b862a648 ("drm/sun4i: Add support for HW scaling to DE2")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206152603.25937-1-jernej.skrabec@siol.net