draw: fix a regression in computing max elt

draw: fix a regression in computing max elt

gl can use elts without setting indices, in which case
our eltMax was set to 0 and always invoking the overflow
condition. So by default set eltMax to maximum, it will
be curbed by draw_set_indexes (if it ever comes) and if
not then it will let gl's glVertexPointer/glDrawArrays
work correctly. Fixes piglit's
triangle-rasterization-overdraw test.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>