i965: Always intel_prepare_render() after invalidating front buffers.

i965: Always intel_prepare_render() after invalidating front buffers.

Fixes glean/texture_srgb, which hit recursive-flush prevention
assertions in vbo_exec_FlushVertices.

This probably hurts the performance of front buffer rendering, but
very few people in their right mind do front buffer rendering.

Fixes Glean's texture_srgb test.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>