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

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>