drm/rockchip: use correct fb width/height

What is passed to drm_fb_helper_fill_var() should be fb_width/fb_height,
rather than the surface size.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>