i915: set SPRITE_POINT_ENABLE bit correctly

i915: set SPRITE_POINT_ENABLE bit correctly

When SPRITE_POINT_ENABLE bit is set, the texture coord would be
replaced, and this is only needed when we called something like
glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE).

And more,  we currently handle varying inputs as texture coord,
we would be careful when setting this bit and set it just when
needed, or you will find the value of varying input is not right
and changed.

Thus we do set SPRITE_POINT_ENABLE bit only when all enabled tex
coord units need do CoordReplace. Or fallback is needed to make
sure the rendering is right.

With handling the bit setup at i915_update_sprite_point_enable(),
we don't need the relative code at i915Enable then.

This patch would _really_ fix the webglc point-size.html test case and
of course, not regress piglit point-sprite and glean-pointSprite
testcase.

NOTE: This is a candidate for stable release branches.

v2: fallback just when all enabled tex coord units need do
    CoordReplace (Eric)
v3: move the sprite point validate code at I915InvalidateState (Eric)
v4: sprite point enable bit update based on _NEW_PROGRAM, too
    add relative _NEW-state comments to show what state is being used(Eric)

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>