drm/mediatek: Fix coverity issue with unintentional integer overflow

1. Instead of multiplying 2 variable of different types. Change to
assign a value of one variable and then multiply the other variable.

2. Add a int variable for multiplier calculation instead of calculating
different types multiplier with dma_addr_t variable directly.

Fixes: 1a64a7af ("drm/mediatek: Fix cursor plane no update")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230907091425.9526-1-jason-jh.lin@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>