drm/sun4i: fix kernel-doc warnings in sunxi_engine.h

drm/sun4i: fix kernel-doc warnings in sunxi_engine.h

Correct the kernel-doc notation, add a missing struct member comment,
and add a missing "Returns:" function comment to eliminate kernel-doc
warnings:

Warning: drivers/gpu/drm/sun4i/sunxi_engine.h:116 Incorrect use of
 kernel-doc format: * @mode_set
Warning: drivers/gpu/drm/sun4i/sunxi_engine.h:125 struct member 'mode_set'
 not described in 'sunxi_engine_ops'
Warning: drivers/gpu/drm/sun4i/sunxi_engine.h:144 struct member 'list'
 not described in 'sunxi_engine'
Warning: drivers/gpu/drm/sun4i/sunxi_engine.h:168 No description found
 for return value of 'sunxi_engine_layers_init'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Link: https://patch.msgid.link/20260219215524.468142-1-rdunlap@infradead.org
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>