drm: Make FB_CORE to be selected if DRM fbdev emulation is enabled

Now that fbdev core has been split in FB_CORE and FB, make the DRM symbol
to select the FB_CORE option if the DRM fbdev emulation layer is enabled.

This allows to disable the CONFIG_FB option if is not needed, which will
avoid the need to explicitly disable each of the legacy fbdev drivers.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719081544.741051-5-javierm@redhat.com