sm501fb: direct color visual does not work

The sm501fb palette code clearly does not handle direct color so change the
driver to use true color visual for 16bpp.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Acked-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>