st/dri: do not create a new context for msaa copy

Commit b77316ad75
    st/dri: always copy new DRI front and back buffers to corresponding MSAA buffers

introduced creating a pipe_context for every call to validate, which is not required
because the callers have a context anyway.

Only exception is egl_g3d_create_pbuffer_from_client_buffer, can someone test if it
still works with NULL passed as context for validate? From examining the code I
believe it does, but I didn't thoroughly test it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: 9.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/include/state_tracker/st_api.h

@@ -342,7 +342,8 @@ struct st_framebuffer_iface
     * the last call might be destroyed.  This behavior might change in the
     * future.
     */
-   boolean (*validate)(struct st_framebuffer_iface *stfbi,
+   boolean (*validate)(struct st_context_iface *stctx,
+                       struct st_framebuffer_iface *stfbi,
                        const enum st_attachment_type *statts,
                        unsigned count,
                        struct pipe_resource **out);