fb: JZ4770: Keep aspect ratio by default

Now, black bars are added around the picture in order to keep the aspect
ratio of the original image.
The fullscreen mode, which doesn't respect the aspect ratio, can be
enabled by writing 0 to
/sys/module/jz4770_fb/parameters/keep_aspect_ratio
fb: JZ4770: Upscale with nearest neighbour with integer ratios

When upscaling with an integer ratio (e.g. 3x), we no upscale with the
nearest neighbour algorithm, which gives the best results.