atmel_lcdfb: fix 16-bpp modes on older SOCs

Fix regression introduced by commit 787f9fd23283 ("atmel_lcdfb: support
16bit BGR:565 mode, remove unsupported 15bit modes") which broke 16-bpp
modes for older SOCs which use IBGR:555 (msb is intensity) rather
than BGR:565.

Use SOC-type to determine the pixel layout.

Tested on at91sam9263 and at91sam9g45.

Cc: <stable@vger.kernel.org>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>