i965/gen7-8: Set up early depth/stencil control appropriately for image load/store.

i965/gen7-8: Set up early depth/stencil control appropriately for image load/store.

v2: Store early fragment test mode in brw_wm_prog_data instead of
    getting it from core mesa data structures (Ken).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>