s3fb: implement fb_get_caps

Implement fb_get_caps().  This will allow the driver to tell upper layer that
it can only support 256 8x16 bitmaps when in tileblitting mode
(var->bits_per_pixel == 0);

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>