drm/amd/pp: Avoid divide-by-zero in fan_ctrl_set_fan_speed_rpm

The minRPM speed maybe equal to zero. so need to check
input RPM not equal to 0, otherwise cause divide-by-zero driver crash.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>