gallium: add a cap to expose whether driver supports mixed color/zs bits

gallium: add a cap to expose whether driver supports mixed color/zs bits

Some hardware can't render to color/depth buffers of mixed bitness. When
that happens a fallback has to happen, but this allows the driver to
express that this isn't an optimal scenario. The purpose of this is to
remove such fbconfigs from the GLX/EGL config list.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>