i965: Fix up the handling of point sprite coordinate replacement.

i965: Fix up the handling of point sprite coordinate replacement.

The code was walking over the regs of pairs of attributes and checking
whether the attribute with a given reg index had point sprite enabled.
So the point sprite setup code was rarely even getting executed.
Instead, we need to determine which channels of a reg need point
sprite coordinate replacement.  In addition, it was multiplying the
attribute by 1/w, when it's supposed to cover (0, 1) in each direction
regardless of w, and it wasn't filling in the Z and W components of
the texcoord as specified.

Fixes piglit point-sprite and the spriteblast demo.  Bug #24431, #22245.