i965: Always allow CPU readback of the scanout on LLC platforms
LLC platforms are magic in that reads from the CPU are always cache coherent, or rather GPU writes that bypass LLC do still invalidate the appropriate cache line. Cc: "17.2" <mesa-stable@lists.freedesktop.org> Reviewed-by:Kenneth Graunke <kenneth@whitecape.org> (cherry picked from commit 49eda75d)