i965 gen7: use SURFACE_STATE fields to select render level/layer

i965 gen7: use SURFACE_STATE fields to select render level/layer

Rather than pointing the surface_state directly at a single
sub-image of the texture for rendering, we now point the
surface_state at the top level of the texture, and configure
the surface_state as needed based on this.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>