st/mesa: add st fp64 support (v7.1)

st/mesa: add st fp64 support (v7.1)

This adds support to the state tracker for
ARB_gpu_shader_fp64.

The details are explained in comments
within the code.

v2 : add double to int/unsigned conversion
v3: handle fp64 consts better
v4: use DRSQ
v4.1: add d2b
v4.2: drop DDIV

v5: split out some prep patches.
v5.1: add some comments.
v5.2: more comments

v6: simplify down the double instruction
    generation loop.

v7: Merge Ilia's two cleanup patches.
v7.1: minor fixups for Ilia patch + cleanups

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>