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>

src/gallium/drivers/softpipe/sp_screen.c

@@ -67,6 +67,7 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    switch (param) {
    case PIPE_CAP_NPOT_TEXTURES:
    case PIPE_CAP_MIXED_FRAMEBUFFER_SIZES:
+   case PIPE_CAP_MIXED_COLOR_DEPTH_BITS:
       return 1;
    case PIPE_CAP_TWO_SIDED_STENCIL:
       return 1;