i965: Only set key->high_quality_derivatives when it matters.

i965: Only set key->high_quality_derivatives when it matters.

This avoids recompiles for shaders that don't use explicit derivatives
when ctx->Hint.FragmentShaderDerivative == GL_NICEST.

For example, GFXBench 5 Aztec Ruins sets the GL_NICEST hint before
compiling any shaders, but none of them use dFdx() or dFdy() - only
implicit derivatives.  This doesn't eliminate any recompiles, but
does eliminate one of the reasons for doing so.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>