drm/amd/display: fix FCLK pstate change underflow

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: fix FCLK pstate change underflow [Why] Currently we set FCLK p-state change watermark calculated based on dummy p-state latency when UCLK p-state is not supported [How] Calculate FCLK p-state change watermark based on on FCLK pstate change latency in case UCLK p-state is not supported