i965/gen6: Add support for layered renderbuffers

i965/gen6: Add support for layered renderbuffers

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.

v2:
 * Use SET_FIELD as suggested by Topi
 * Simplify min_array_element assignment as suggested by Topi
v3:
 * Use irb->layer_count for depth instead of rb->Depth
 * Make gl_target const
 * depth - 1, not depth
v4:
 * Merge in dd43900b & b875f39e fixes to prevent 3D texture piglit
   regressions

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>