i965: Fix the W value of deprecated pointcoords on pre-gen6.

i965: Fix the W value of deprecated pointcoords on pre-gen6.

When you didn't have a texcoord array bound (or a non-1 current w
attrib), we were telling the fragment shader that it could just use "1"
instead of doing expensive pre-gen6 math to invert it.  If you drew the
point with a non-1 W value, then you'd get the right size (since all the
vertex computations worked), but we'd mis-interpolate the coordinate
across the face.

Fixes the mesa pointsprite demo on GM45.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30232
Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>
Note: This is a candidate for the stable branches.