lxfb: properly alloc cmap in all cases and don't leak the memory

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..

[dilinger@debian.org: dropped unnecessary code and clean up patch]
[dilinger@debian.org: add error checking and handling]
Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>