vbo: introduce vbo_sizeof_ib_type() function

vbo: introduce vbo_sizeof_ib_type() function

introduce vbo_sizeof_ib_type() function to return the index data type
size. I see some place use switch(ib->type) to get the index data type,
which is sort of duplicate.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>