gxfb: properly alloc cmap and plug cmap leak

We weren't properly allocating the cmap for depths greater than 8bpp,
which caused pain for things like DirectFB.  Also, we never freed the cmap
memory upon module unload..

Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: Marco La Porta <marco-laporta@tiscali.it>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>