drm/i915: Round TMDS clock to nearest

Use round-to-nearest behavour when calculating the TMDS clock.
Matches what we do for most other clock related things.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-18-ville.syrjala@linux.intel.com