drm/radeon: use frac fb div on RS780/RS880

commit 411678288d61ba17afe1f8afed92200be6bbc65d upstream.

Monitors seem to prefer it.  Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=37696

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Add to pll->flags, not radeon_crtc->pll_flags]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>