i965: Remember to call intel_prepare_render() before blitting.

i965: Remember to call intel_prepare_render() before blitting.

Otherwise, blits to the window system buffer may cause crashes,
since dst_irb->mt may be NULL.

This code is lifted straight out of brw_blorp_framebuffer()'s
try_blorp_blit() helper.

Fixes crashes in Piglit's fbo-sys-blit on systems without BLORP.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65919
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <idr@freedesktop.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Cc: "9.2" <mesa-stable@lists.freedesktop.org>