ui/gtk: Remove NULL checks in gd_switch

ui/gtk: Remove NULL checks in gd_switch

c821a58ee7 ("ui/console: Pass placeholder surface to display")
eliminated the possibility that NULL is passed as surface to
dpy_gfx_switch and removed some NULL checks from gd_switch, but the
removal was not thoroughly. Remaining NULL checks were confusing for
Coverity and probably also for humans. This change removes those NULL
checks.

Reported-by: Coverity (CID 1448421)
Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210308140713.17901-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>