drm/nva3/clk: better pll calculation when no fractional fb div available

The core/mem/shader clocks don't support the fractional feedback divider,
causing our calculated clocks to be off by quite a lot in some cases.  To
solve this we will switch to a search-based algorithm when fN is NULL.

For my NVA8 at PL3, this actually generates identical cooefficients to
the binary driver.  Hopefully that's a good sign, and that does not
break VPLL calculation for someone..

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>