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

Commit b77316ad7594f
    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>
(cherry picked from commit b217d48364f368f541e53006af5dd56f664be24d)