drm/radeon/kms: use frac fb div on APUs

Seems to be more stable on certain monitors.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=48880

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/atombios_crtc.c

@@ -575,6 +575,9 @@ static u32 atombios_adjust_pll(struct drm_crtc *crtc,
 
 		if (rdev->family < CHIP_RV770)
 			pll->flags |= RADEON_PLL_PREFER_MINM_OVER_MAXP;
+		/* use frac fb div on APUs */
+		if (ASIC_IS_DCE41(rdev) || ASIC_IS_DCE61(rdev))
+			pll->flags |= RADEON_PLL_USE_FRAC_FB_DIV;
 	} else {
 		pll->flags |= RADEON_PLL_LEGACY;