drm/rockchip: remove existing generic drivers to take over the device

There are drivers that register framebuffer devices very early in the boot
process and make use of the existing framebuffer as setup by the firmware.

If one of those drivers has registered a fbdev, then the fallback fbdev of
the DRM driver won't be bound to the framebuffer console. To avoid that,
remove any existing generic driver and take over the graphics device.

By doing that, the fb mapped to the console is switched correctly from the
early fbdev to the one registered by the rockchip DRM driver:

    [   40.752420] fb0: switching to rockchip-drm-fb from EFI VGA

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210516074833.451643-1-javierm@redhat.com