vbo: fix attr reset

vbo: fix attr reset

In bc4e0c4 (vbo: Use a bitmask to track the active arrays in vbo_exec*.)
we stopped looping over all the attributes and resetting all slots.
Which exposed an issue in vbo_exec_bind_arrays() for handling GENERIC0
vs. POS.

Split out a helper which can reset a particular slot, so that
vbo_exec_bind_arrays() can re-use it to reset POS.

This fixes an issue with 0ad (and possibly others).

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>