i965: Bump the initial program cache size from 4kB to 16kB.

i965: Bump the initial program cache size from 4kB to 16kB.

Our initial size of 4kB is way too small to do anything useful, so we
end up growing it at least a few times.  We may as well start it larger.

Some data points:

- Dinoshade (from Mesa Demos): hit 8kB.
- Chromium 60: hit 16kB after browsing a few things in Google Docs.
- GFXBench4 TRex/Manhattan 3.1: hit 128kB
- Unigine Valley 1.0: hit 512kB

It might make sense to start it even larger.

Acked-by: Matt Turner <mattst88@gmail.com>