drm/ast: 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: Dave Airlie <airlied@redhat.com>
Cc: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Cc: YueHaibing <yuehaibing@huawei.com>
Cc: Sam Bobroff <sbobroff@linux.ibm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-6-daniel.vetter@ffwll.ch