gallium: Add MULTISAMPLE_Z_RESOLVE cap

Resolving a multisampled depth texture into
a single sampled texture is supported on >= SM4.1
hw. It is possible some previous hw support it.

The ability was tested on radeonsi and nvc0.
Apparently is is also supported for radeon >= r700.

This patch adds the MULTISAMPLE_Z_RESOLVE cap and
add it to the drivers. It is advertised for drivers
for which it is sure the ability is supported.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Axel Davy <axel.davy@ens.fr>

src/gallium/docs/source/screen.rst

@@ -243,6 +243,9 @@ The integer capabilities:
   this, at least BASEVERTEX should be exposed separately too).
 * ``PIPE_CAP_POLYGON_OFFSET_CLAMP``: If true, the driver implements support
   for ``pipe_rasterizer_state::offset_clamp``.
+* ``PIPE_CAP_MULTISAMPLE_Z_RESOLVE``: Whether the driver supports blitting
+  a multisampled depth buffer into a single-sampled texture (or depth buffer).
+  Only the first sampled should be copied.
 
 
 .. _pipe_capf: