drm/sun4i: backend: fix error pointer dereference

drm/sun4i: backend: fix error pointer dereference

The function drm_atomic_get_plane_state() can return an error pointer
and is not checked for it. Add error pointer check.

Detected by Smatch:
drivers/gpu/drm/sun4i/sun4i_backend.c:496 sun4i_backend_atomic_check() error:
'plane_state' dereferencing possible ERR_PTR()

Fixes: 96180dde23b79 ("drm/sun4i: backend: Add a custom atomic_check for the frontend")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Link: https://patch.msgid.link/20260217014801.60760-1-ethantidmore06@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>