drm/radeon: only apply the SS fractional workaround to RS[78]80

Looks like some RV6xx have problems with that.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=97099

Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/radeon/atombios_crtc.c

···
 			if (radeon_crtc->ss.refdiv) {
 				radeon_crtc->pll_flags |= RADEON_PLL_USE_REF_DIV;
 				radeon_crtc->pll_reference_div = radeon_crtc->ss.refdiv;
-				if (rdev->family >= CHIP_RV770)
+				if (ASIC_IS_AVIVO(rdev) &&
+				    rdev->family != CHIP_RS780 &&
+				    rdev->family != CHIP_RS880)
 					radeon_crtc->pll_flags |= RADEON_PLL_USE_FRAC_FB_DIV;
 			}
 		}