drm/amd/display: Revert to old formula in set_vtg_params

[Why]
New formula + cursor change causing underflow
on certain configs

[How]
Rever to old formula
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_optc.c

@@ -299,6 +299,7 @@ void optc1_set_vtg_params(struct timing_generator *optc,
 	uint32_t asic_blank_end;
 	uint32_t v_init;
 	uint32_t v_fp2 = 0;
+	int32_t vertical_line_start;
 
 	struct optc *optc1 = DCN10TG_FROM_TG(optc);
 
@@ -315,8 +316,9 @@ void optc1_set_vtg_params(struct timing_generator *optc,
 			patched_crtc_timing.v_border_top;
 
 	/* if VSTARTUP is before VSYNC, FP2 is the offset, otherwise 0 */
-	if (optc1->vstartup_start > asic_blank_end)
-		v_fp2 = optc1->vstartup_start - asic_blank_end;
+	vertical_line_start = asic_blank_end - optc1->vstartup_start + 1;
+	if (vertical_line_start < 0)
+		v_fp2 = -vertical_line_start;
 
 	/* Interlace */
 	if (REG(OTG_INTERLACE_CONTROL)) {