drm/i915: Make M/N checks non-fuzzy

Now that we no longer fuzz M/N during fastset these should
match exctly.

In order to get a match with what the BIOS does we need to round
M/N down. And we do the opposite rounding when doing the readback.
That gets us pretty much the same thing back.

There can still be slight rounding differences between FDI M/N
vs. the DPLL output so we allow for tiny deviation in
intel_pipe_config_sanity_check().

v2: Tweak rounding/sanity check stuff a bit

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-10-ville.syrjala@linux.intel.com