tgafb: fix cmap memory leak

Fix cmap leak when register_framebuffer fails.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Acked-by: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>