This patch fixes depth stencil ops on MX6S GC880 and MX6Q GC2000.
The following dEQPs now pass:
  dEQP-GLES2.functional.fragment_ops.depth_stencil.stencil_depth_funcs.*
  dEQP-GLES2.functional.fragment_ops.depth_stencil.stencil_ops.*
which is roughly 600 fixed dEQP tests.

The problem is that if the front-facing stencil has a value mask 0x00 and
the back-facing stencil has some non-zero value mask, then the stencil part
of the depth stencil buffer is written with 0x00 unconditionally. The blob
replicates the value mask of the back-facing stencil to the value mask of
the front-facing stencil to achieve correct rendering, replicate the same
behavior here.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <mesa/mesa!4275>