If the downscaling fails and we end up with a best_depth of 0,
then ignore it.

This actually works around a cascade of failure, but it the
simplest fix for now.

The scaling patch broke the udl driver, as the udl driver doesn't
expose planes at all, so gets the two default 32-bit formats, but
the udl driver then ask for 16bpp fbdev, and the scaling code falls
over.

This fixes the udl driver since the scaled depth support was added.

Fixes: f4bd542b ("drm/fb-helper: Scale back depth to supported maximum")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315014621.21816-2-airlied@gmail.com