drm/radeon/dce32+: use fractional fb dividers for high clocks

commit a02dc74b317d78298cb0587b9b1f6f741fd5c139 upstream.

Fixes flickering with some high res montiors.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: use pll->flags instead of radeon_crtc->pll_flags]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>