i965/fs: Rework the persample shading key/prog_data bits

i965/fs: Rework the persample shading key/prog_data bits

This commit reworks and simplifies the way we handle persample shading in
the shader key and prog_data.  The previous approach had three different
key bits that had slightly different and hard-to-decern meanings while the
new bits are far more clear.  This commit changes it to two easily
understood bits that communicate everything we need:

 1) key->persample_interp: means that the user has requested persample
    interpolation through the API.  This is equivalent to having
    SAMPLE_SHADING enabled and having MIN_SAMPLE_SHADING_VALUE set high
    enough that you actually get multiple per-sample invocations.

 2) key->multisample_fbo: means that the shader will be running on an
    actual multi-sampled framebuffer.

This commit also adds a new "persample_dispatch" bit to prog_data which
indicates that the shader should be run in persample mode.  This way the
state setup code doesn't have to look at the fragment program or GL state
and can just pull that data out of the prog_data.

In theory, this shuffle could mean more recompiles.  However, in practice,
we were shoving enough state into the key before that we were probably
hitting a recompile on every per-sample shader anyway.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>