MIPS: JZ4770: Move jz4770_fb.h header to <linux/platform_data/jz4770.h>

It only contains the structure that defines the format of the platform
data, so that's where it belongs.
fb: JZ4770: Add a parameter to enable/disable downscaling

And disable downscaling by default.
This switch is useful for applications which support higher resolutions
and will use them by default; when downscaling is disabled, it forces
them to render to the panel's native resolution.