drm/tinydrm: Use drm_fbdev_generic_setup()

Make full use of the generic fbdev client.

Cc: David Lechner <david@lechnology.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703160354.59955-8-noralf@tronnes.org