drm/amd/display: Optimize vstartup position for AS-SDP

[Why]
In current design, the vstartup position is adjusted to
vblank start position when AS-SDP is enabled.
However when the vblank length is too big, it may over
vstartup boundary.

[How]
To adjust vstartup position to 1 line before vsync position.
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Robin Chen <robin.chen@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml/dcn20/dcn20_fpu.c

@@ -1132,7 +1132,8 @@ static void dcn20_adjust_freesync_v_startup(
 					patched_crtc_timing.v_addressable -
 					patched_crtc_timing.v_border_top;
 
-	newVstartup = asic_blank_end + (patched_crtc_timing.v_total - asic_blank_start);
+	/* The newVStartUp is 1 line before vsync point */
+	newVstartup = asic_blank_end + 1;
 
 	*vstartup_start = ((newVstartup > *vstartup_start) ? newVstartup : *vstartup_start);
 }