arm64: defconfig: DRM_DUMB_VGA_DAC: follow changed config symbol name

arm64: defconfig: DRM_DUMB_VGA_DAC: follow changed config symbol name

This occurrence wasn't changed in the original rename commit.

Fixes commit 3ec63f9a53c42 ("drm/bridge: dumb-vga-dac: Rename driver to
simple-bridge").

Link: https://lore.kernel.org/r/20200427134003.45188-2-max.krummenacher@toradex.com
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>