freedreno: Improve the pi approximations in trig lowering.

When comparing our sin/cos behavior to the closed source driver, I
noticed that we were off by a bit (or, in the case of 1/2pi, 3 bits).

Fixes:
dEQP-GLES3.functional.shaders.random.trigonometric.vertex.52
dEQP-GLES3.functional.shaders.random.all_features.vertex.0

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>