i965: Replace DEPTH_STENCIL_STATE with Gen8's 3DSTATE_WM_DEPTH_STENCIL.

i965: Replace DEPTH_STENCIL_STATE with Gen8's 3DSTATE_WM_DEPTH_STENCIL.

v2: Use stencil->_WriteEnabled instead of setting
    GEN8_WM_DS_STENCIL_BUFFER_WRITE_ENABLE twice (suggested by Eric).

v3: Mask stencil->WriteMask and stencil->ValueMask with 0xff.  The field
    is only 8-bits, so we'd trip the new SET_FIELD assertion when core
    Mesa gave us a value like 0xFFFFFFFF.  The Gen7 code uses structure
    field widths to implicitly do this truncation.  Fixes Piglit tests.

v4: Use uint32_t for dw1/dw2, not uint8_t.  Worst. Typo. Ever.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> [v2]