gallium: better flush logic in draw module

gallium: better flush logic in draw module

This is the other half of Keith's draw/flush patch.

There are now 5 flush flags to control what's flushed (post-xform vertex
cache, prim cache, vbuf, etc).

The gears slow-down in this part of the patch was due to the cull stage not
getting invoked.  It was unconditional before, but is now gated by 'need_det'.
But it also needs to be gated by draw->rasterizer->cull_mode.  Gears uses
back-face culling.