drm/cirrus: Use drm_fb_helper_fill_info

Should not result in any changes.

v2: Rebase

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-7-daniel.vetter@ffwll.ch