drm/radeon: use fixed PPL ref divider if needed

Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -880,6 +880,11 @@ void radeon_compute_pll_avivo(struct radeon_pll *pll,
 		ref_div_min = pll->reference_div;
 	else
 		ref_div_min = pll->min_ref_div;
+
+	if (pll->flags & RADEON_PLL_USE_FRAC_FB_DIV &&
+	    pll->flags & RADEON_PLL_USE_REF_DIV)
+		ref_div_max = pll->reference_div;
+	else
 		ref_div_max = pll->max_ref_div;
 
 	/* determine allowed post divider range */