media: platform: mtk-mdp3: Constify buffer passed to mdp_vpu_sendmsg()

mdp_vpu_sendmsg() passes the buffer to scp_ipi_send(), which takes now
pointer to const, so adjust this interface as well for increased code
safety and code readability.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260317-rpmsg-send-const-v3-5-4d7fd27f037f@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

Author: krzk

drivers/media/platform/mediatek/mdp3/mtk-mdp3-vpu.c

@@ -163,7 +163,7 @@ void mdp_vpu_unregister(struct mdp_dev *mdp)
 }
 
 static int mdp_vpu_sendmsg(struct mdp_vpu_dev *vpu, enum scp_ipi_id id,
-			   void *buf, unsigned int len)
+			   const void *buf, unsigned int len)
 {
 	struct mdp_dev *mdp = vpu_to_mdp(vpu);
 	unsigned int t = MDP_VPU_MESSAGE_TIMEOUT;