Comment # 6 on bug 77673 from Christian König

Please try the attached patch.

We now always use the fixed reference divider if both RADEON_PLL_USE_REF_DIV
and RADEON_PLL_USE_FRAC_FB_DIV are set.

If only RADEON_PLL_USE_REF_DIV is set then we only use the fixed value as
minimum limit.