fbdev: fix frame buffer devices menu

Commit f6014419 ("imxfb: add support for
i.MX25:) has inserted the symbol HAVE_FB_IMX, which does not depend on FB
after the menuconfig FB.  This breaks the menu, presenting most of the
drivers outside of it, when using menuconfig.

Moving the symbol to the start of the file, just like HAVE_FB_ATMEL, fixes
the problem without breaking it for iMX25 configurations (tested with
ARCH=arm, no build).

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>