i965: Drop intel_check_front_buffer_rendering().

i965: Drop intel_check_front_buffer_rendering().

This was being applied in a subset of the places that
intel_prepare_render() was called, to set the same flag that
intel_prepare_render() was setting.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>