pwm: mtk-disp: Fix overflow in period and duty calculation

Current calculation for period and high_width may have 64-bit overflow.
state->period and rate are u64. rate * state->period will overflow.

clk_div = div_u64(rate * state->period, NSEC_PER_SEC)
period = div64_u64(rate * state->period, div);
high_width = div64_u64(rate * state->duty_cycle, div);

This patch is to resolve it by using mul_u64_u64_div_u64().

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>