drm/radeon: fix PLLs on RS880 and older v2

This is a workaround for RS880 and older chips which seem to have
an additional limit on the minimum PLL input frequency.

v2: fix signed/unsigned warning

bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=91861
https://bugzilla.kernel.org/show_bug.cgi?id=83461

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/radeon_display.c

@@ -960,6 +960,9 @@ void radeon_compute_pll_avivo(struct radeon_pll *pll,
 	if (pll->flags & RADEON_PLL_USE_FRAC_FB_DIV &&
 	    pll->flags & RADEON_PLL_USE_REF_DIV)
 		ref_div_max = pll->reference_div;
+	else if (pll->flags & RADEON_PLL_PREFER_MINM_OVER_MAXP)
+		/* fix for problems on RS880 */
+		ref_div_max = min(pll->max_ref_div, 7u);
 	else
 		ref_div_max = pll->max_ref_div;