drm/amd/display: Fix psr static frames calculation

[Why]
Driver crash with psr feature enabled due to divide-by-zero error.
This is a regression after rework to calculate static screen frame
number entry time.

[How]
Correct order of operations to avoid divide-by-zero.
Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -8408,7 +8408,6 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
 	/* Calculate number of static frames before generating interrupt to
 	 * enter PSR.
 	 */
-	unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
 	// Init fail safe of 2 frames static
 	unsigned int num_frames_static = 2;
 
@@ -8423,8 +8422,10 @@ bool amdgpu_dm_psr_enable(struct dc_stream_state *stream)
 	 * Calculate number of frames such that at least 30 ms of time has
 	 * passed.
 	 */
-	if (vsync_rate_hz != 0)
+	if (vsync_rate_hz != 0) {
+		unsigned int frame_time_microsec = 1000000 / vsync_rate_hz;
 		num_frames_static = (30000 / frame_time_microsec) + 1;
+	}
 
 	params.triggers.cursor_update = true;
 	params.triggers.overlay_update = true;