i965: Add 2x MSAA support to Broadwell fast clear code.

According to the cited documentation section (but in the newer docs),
x_scaledown is the same for 2x and 4x MSAA.

+47 piglits.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83081
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.3" <mesa-stable@lists.freedesktop.org>

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

@@ -282,6 +282,7 @@ get_fast_clear_rect(struct brw_context *brw, struct gl_framebuffer *fb,
        * factor is 2 vertically and either 2 or 8 horizontally.
        */
       switch (irb->mt->num_samples) {
+      case 2:
       case 4:
          x_scaledown = 8;
          break;