drm/i915: Don't overestimate 4:2:0 link symbol clock

With 4:2:0 output the LS clock can be half of what it is with 4:4:4.
Make that happen.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125851.3275-1-ville.syrjala@linux.intel.com


Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Tested-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>