drm: shmobile: Add missing call to drm_fbdev_generic_setup()

Set up generic fbdev emulation, to enable support for the Linux console.

Use 16 as the preferred depth, as that is a good compromise between
colorfulness and resource utilization, and the default of the fbdev
driver.

Suggested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/c836938112fda21762bc9eb2741ccd0cbf1197ef.1684854992.git.geert+renesas@glider.be