i965: compute DDX in a subspan based only on top row

Consider only the top-left and top-right pixels to approximate DDX in a 2x2
subspan, unless the application requests a more accurate approximation via
GL_FRAGMENT_SHADER_DERIVATIVE_HINT or this optimization is disabled from the
new driconf option disable_derivative_optimization.

This results in a less accurate approximation.  However, it improves the
performance of Xonotic with Ultra settings by 24.3879% +/- 0.832202% (at 95.0%
confidence) on Haswell.  No noticeable image quality difference observed.

The improvement comes from faster sample_d.  It seems, on Haswell, some
optimizations are introduced to allow faster sample_d when all pixels in a
subspan have the same derivative.  I considered SAMPLE_STATE too, which allows
one to control the quality of sample_d on Haswell.  But it gave much worse
image quality without giving better performance comparing to this change.

No piglit quick.tests regression on Haswell (tested with v1).

v2: better guess for precompile program key

Signed-off-by: Chia-I Wu <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Reviewed-by: Chris Forbes <[email protected]>

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