softpipe: allocate tile data on demand

softpipe: allocate tile data on demand

Changes in v2:
- Invalidate last_tile_addr on any change, fixing regressions
- Correct coding style

Currently softpipe ends up allocating more than 200 MB of memory
for each context due to the tile caches.

Even worse, this memory is all explicitly cleared, which means that the
kernel must actually back it with physical RAM right away.

This change allocates tile memory on demand.

Signed-off-by: Brian Paul <brianp@vmware.com>