drm/i915: Fix MST oops due to MSA changes

The MSA MISC computation now depends on the connector state, and
we do it from the DDI .pre_enable() hook. All that is fine for
DP SST but with MST we don't actually pass the connector state
to the dig port's .pre_enable() hook which leads to an oops.

Need to think more how to solve this in a cleaner fashion, but
for now let's just add a NULL check to stop the oopsing.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Fixes: 0c06fa15

 ("drm/i915/dp: Add support of BT.2020 Colorimetry to DP MSA")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015190538.27539-1-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>