drm/i915: Use port clock to set correct N value

Use port_clock to check the clock values in n/cts lookup table instead
of crtc_clock. As port_clock is already adjusted based on color mode set
(8 bit or deep color), this will help in checking clock values for deep
color modes from n/cts lookup table.

Signed-off-by: Aditya Swarup <aditya.swarup@intel.com>
Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627220708.31700-1-aditya.swarup@intel.com