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

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>