vbo: introduce vbo_get_minmax_indices function

vbo: introduce vbo_get_minmax_indices function

Introduce vbo_get_minmax_indices() function to handle the min/max index
computation for nr_prims(>= 1). The old code just compute the first
prim's min/max index; this would results an error rendering if user
called functions like glMultiDrawElements(). This patch servers as
fixing this issue.

As when nr_prims = 1, we can pass 1 to paramter nr_prims, thus I made
vbo_get_minmax_index() static.

v2: per Roland's suggestion, put the indices address compuation into
    vbo_get_minmax_index() instead.

    Also do comination if possible to reduce map/unmap count

v3: per Brian's suggestion, use a pointer for start_prim to avoid
    structure copy per loop.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>