i965: Temporarily disable async mappings on non-LLC

i965: Temporarily disable async mappings on non-LLC

Fixes regressions from commits e0a9b261e593 and a16355d67d92 by
neutering async mappings on non-LLC to be synchronous, like they were
before those two commits. :(

The failing tests include

piglit-test piglit.spec.nv_primitive_restart.primitive-restart-vbo_index_only
piglit-test piglit.spec.nv_primitive_restart.primitive-restart-vbo_combined_vertex_and_index
piglit-test piglit.spec.nv_primitive_restart.primitive-restart-vbo_separate_vertex_and_index
piglit-test piglit.spec.nv_primitive_restart.primitive-restart-vbo_vertex_only
piglit-test piglit.spec.arb_pixel_buffer_object.texsubimage-unpack pbo