drm/sti: avoid potential dereference of error pointers

commit 831214f770 upstream.

The return value of drm_atomic_get_crtc_state() needs to be
checked. To avoid use of error pointer 'crtc_state' in case
of the failure.

Cc: stable@vger.kernel.org
Fixes: dd86dc2f9a ("drm/sti: implement atomic_check for the planes")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20240913090412.2022848-1-make24@iscas.ac.cn
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/sti/sti_cursor.c

@@ -200,6 +200,9 @@ static int sti_cursor_atomic_check(struct drm_plane *drm_plane,
 		return 0;
 
 	crtc_state = drm_atomic_get_crtc_state(state, crtc);
+	if (IS_ERR(crtc_state))
+		return PTR_ERR(crtc_state);
+
 	mode = &crtc_state->mode;
 	dst_x = new_plane_state->crtc_x;
 	dst_y = new_plane_state->crtc_y;