[intel] Re-allocate backing buffer objects on BufferData.

[intel] Re-allocate backing buffer objects on BufferData.

This may allow better concurrency (noop in openarena performance now), but is
also important for the previous commit -- otherwise, we may end up with
BufferData, draw_prims, BufferData and the draw_prims would use the new VBO
data instead of old.  This could still occur with user-supplied VBOs and poor
use of MapBuffer without BufferData.