i915: Remember to call intel_prepare_render() before blitting

i915: Remember to call intel_prepare_render() before blitting

Bring over the following fix from i965:
 commit fb3d62fe3d4fc40ba4ad9804d8b6f451316c9ae2
 Author: Kenneth Graunke <kenneth@whitecape.org>
 Date:   Tue Aug 6 14:36:09 2013 -0700

    i965: Remember to call intel_prepare_render() before blitting.

Fixes a crash in the following piglit tests:
 bin/fbo-sys-blit -auto
 bin/fbo-sys-sub-blit -auto

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>