softpipe: Shrink context size.

softpipe: Shrink context size.

- each softpipe_tex_tile_cache 50*64*64*4*4 = 3,276,800 bytes
- each softpipe_context has 3*32 softpipe_tex_tile_cache, i.e, each softpipe
  context is 314,572,800 bytes, i.e, 300MB

That is, in a 32bits process (around 3GB virtual memory max), we can
only fit 10 contexts.

This change is a short-term hack to shrink the context size.  Longer
term we'll need to change how the texture cache works.

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