i965/gen7: Fix the /* ignored */ comment on constant surface setup.

It turns out this field *is* used, and it's the stride between samples
from the buffer.  Discovered during TBO debugging.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/gen7_wm_surface_state.c

@@ -164,7 +164,7 @@ gen7_create_constant_surface(struct brw_context *brw,
    surf->ss2.width = w & 0x7f;            /* bits 6:0 of size or width */
    surf->ss2.height = (w >> 7) & 0x1fff;  /* bits 19:7 of size or width */
    surf->ss3.depth = (w >> 20) & 0x7f;    /* bits 26:20 of size or width */
-   surf->ss3.pitch = (16 - 1); /* ignored */
+   surf->ss3.pitch = (16 - 1); /* stride between samples */
    gen7_set_surface_tiling(surf, I915_TILING_NONE); /* tiling now allowed */
 
    if (brw->intel.is_haswell) {