drm/armada: Hide fbdev support behind config option

Only build armada's fbdev emulation if CONFIG_DRM_FBDEV_EMULATION
has been enabled. No functional changes, but allows to build armada
without fbdev.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230330073046.7150-3-tzimmermann@suse.de