ui/cocoa.m: Fix recent compile breakage

Fix failures to compile introduced by recent console commits
1dbfa00503, 81c0d5a6) which removed is_graphic_console() and
vga_hw_update() without updating the cocoa UI backend to match.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>