i965/Gen6-7: Fix point sprites with PolygonMode(GL_POINT)

i965/Gen6-7: Fix point sprites with PolygonMode(GL_POINT)

This was an oversight in the original patch. When PolygonMode is
used, then front faces, back faces, or both may be rendered as
points and are affected by point sprite state.

Note that SNB/IVB can't actually be fully conformant here, for
a legacy context -- we don't have separate sets of pointsprite
enables for front and back faces. Haswell ignores pointsprite
state correctly in hardware for non-point rasterization, so can
do this correctly, but it doesn't seem worth it.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86764
Reviewed-by: Matt Turner <mattst88@gmail.com>