sgivwfb: Make use of ARRAY_SIZE.

Cleanup the usage of DBE_VT_SIZE since the kernel already defines the
same macro for the same propose.

Also clean up a surrounding whitespaces.

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>