i965/gen7: Add support for gl_PointCoord.

This is exactly analogous to Eric's Gen6 change in commit
6861a70177.  His explanation:

"This is just like PointSprite overrides, but it's always on for that
 attribute."

Fixes glsl-fs-pointcoord and gtf/point_sprites.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

NOTE: This is a candidate for the 7.11 branch.

src/mesa/drivers/dri/i965/gen7_sf_state.c

@@ -79,6 +79,9 @@ upload_sbe_state(struct brw_context *brw)
 	 dw10 |= (1 << input_index);
       }
 
+      if (attr == FRAG_ATTRIB_PNTC)
+	 dw10 |= (1 << input_index);
+
       /* The hardware can only do the overrides on 16 overrides at a
        * time, and the other up to 16 have to be lined up so that the
        * input index = the output index.  We'll need to do some