drm/amd/display: make PSR static screen entry within 30 ms

[Colin Ian King]

Hi,

Static analysis with Coverity has detected a division by zero in the
following commit:

commit 5b5abe9526073ccbf3032d27b5864520829cdd9c
Author: Anthony Koo <Anthony.Koo at amd.com>
Date:   Mon Dec 9 17:26:34 2019 -0500

    drm/amd/display: make PSR static screen entry within 30 ms

Specifically:

       unsigned int vsync_rate_hz = 0;
       struct dc_static_screen_params params = {0};
       /* Calculate number of static frames before generating interrupt to
        * enter PSR.
        */
       unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;

vsync_rate_hz is zero, and frame_time_microsec is being assigned a value
that is being divided by zero.  I'm not sure why this is coded this way
and not sure what the fix is, hence I'm reporting the issue.

Colin