fbdev: add tile operation to get the maximum length of the map

Add a tile method, fb_get_tilemax(), that returns the maximum length of
the tile map (or font map).  This is needed by s3fb which can only handle
256 characters.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>