i915: Enable point sprite coordinate generation

i915: Enable point sprite coordinate generation

Support still isn't completely correct, but it's better.  piglit
point-sprite now passes.  However, glean's pointSprite test fails.  In
that test the texture on the sprite is somehow inverted as though
GL_POINT_SPRITE_COORD_ORIGIN were set to GL_LOWER_LEFT.  i915 hardware
shouldn't be able to do that!

I believe there are also problems when not all texture units have
GL_COORD_REPLACE set.  The hardware enable seems to be all or nothing.

Fixes bug #25313.