i965: Reenable the fragment shader precompile.

Precompiling the shader at link time often allows us to avoid compiling
it at the first use.  This moves the expensive compilation and
optimization process to game or level load time, rather than at draw
time, where we really can't avoid any cycles and don't want to risk
stalling the GPU.

The downside is that we have to guess the non-orthagonal state the
program will have set when it draws with the shader.  Previously, we
guessed wrong for nearly every shader, so it wasn't useful.  With the
recent SamplerUnits rework and this series, we've either eliminated
state or made smarter guesses, and usually get it right now.

In the L4D2 time demo, I now have 39 fragment shader recompiles and no
vertex shader recompiles.  Before this series and the SamplerUnits
rework, I had 206 fragment shader recompiles and 192 vertex shader
recompiles.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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