drm/mediatek: Fix coverity issue with unintentional integer overflow

In the Linux kernel, the following vulnerability has been resolved: 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.