drm/cirrus: add plane setup

Commit "f4bd542b drm/fb-helper: Scale back depth to supported maximum"
uncovered a bug in the cirrus driver.  It must create its own primary
plane, using the correct format list, depending on the bpp module
parameter, so it is consistent with mode_config->preferred_depth.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190204110131.21467-1-kraxel@redhat.com