LCD: support 8bpp BMPs on 16bpp displays

This patch also simplifies some ifdefs in lcd.c, introduces a generic
vidinfo_t, which new drivers are encouraged to use and old drivers to switch
over to.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>