drm/amd/display: Store gem objects for planes 1-3

Also do the proper validation which was missed.

Besides the missing validation, not storing the objects in the
framebuffer led to wrong results in the getfb2 ioctl.

Note that this should really have been done for multi-plane
formats like NV12 already before modifiers landed.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>