i965/draw: Use worst-case VBO bounds if brw->num_instances == 0

i965/draw: Use worst-case VBO bounds if brw->num_instances == 0

Previously, we only handled the "I don't know what's going on" case for
things with InstanceDivisor == 0.  However, in the DrawIndirect case we can
get num_instances == 0 and we don't know what's going on with the instanced
ones either.  This commit makes the worst-case bound the default and then
conservatively tightens the bound.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>