drm/rockchip: pass 0 to drm_fbdev_generic_setup()

Allow drm_fbdev_generic_setup() to pick the default bpp value for the
framebuffer.

This has no functional impact because the default is 32, given that
mode_config.preferred_depth is not set for Rockchip.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211101114622.813536-1-john@metanate.com