i965: Implement another VF cache invalidate workaround on Gen8+.

...and provide a better citation for the existing one.

v2:
- Apply the workaround to Gen8 too, as intended (caught by Topi).
- Restructure to add bits instead of an extra flush (based on a similar
  patch by Rafael Antognolli).

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
(cherry picked from commit 8d486714)
[Andres Gomez: brw->gen not yet dropped in favor of devinfo->gen]
Signed-off-by: Andres Gomez <agomez@igalia.com>

Conflicts:
	src/mesa/drivers/dri/i965/brw_pipe_control.c

Squashed with:

i965: Revert Gen8 aspect of VF PIPE_CONTROL workaround.

This apparently causes hangs on Broadwell, so let's back it out for now.
I think there are other PIPE_CONTROL workarounds that we're missing.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103787


(cherry picked from commit a01ba366)
[Andres Gomez: brw->gen not yet dropped in favor of devinfo->gen]
Signed-off-by: Andres Gomez <agomez@igalia.com>

Conflicts:
	src/mesa/drivers/dri/i965/brw_pipe_control.c