lxfb: drop dead declarations from header

We never sent the gamma stuff upstream, and don't really care about it.
However, lx_[gs]_et_gamma prototypes snuck into lxfb.h anyways; there are
no definitions for them.  Drop the dead code.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>