staging: sm7xxfb: minor maintenance on sm7xx_vga_setup

This patch keeps code related to sm7xx_vga_setup closed. It is useful to
understand/maintain the logic behind sm7xx_vga_setup with a simple look.

Tested with SM712.

Signed-off-by: Javier M. Mellid <jmunhoz@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>