imxfb: add 18 bit support

v2: As pointed out by Hans J. Koch we have to claim we can do 24bit to
    make software like X work. We are lucky on i.MX that 18bit support
    has the necessary gaps in the fields to do so.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
1 file changed