Remove references to non-existent fbmon_valid_timings()

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: James Simmons <jsimmons@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>