llvmpipe: Unmapping vertex/index buffers does NOT flush draw module anymore.

llvmpipe: Unmapping vertex/index buffers does NOT flush draw module anymore.

Not since 6094e79f4e3350d123c7532b1c73faa60834a62d.

Drivers now need to flush draw module explicitely (which explains why
all those previous commits adding draw_flushes calls were necessary).

This is a good thing, but it's tricky to get this right in face of user buffers
(it's not even clear who has the responsibility to flush when a user buffer
is seen -- statetracker or pipe driver), so just force flush (temporarily)
since it's not a bottleneck now.