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: 0c06fa1 ("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>

drivers/gpu/drm/i915/display/intel_ddi.c

@@ -1794,8 +1794,10 @@ void intel_ddi_set_dp_msa(const struct intel_crtc_state *crtc_state,
 	 * of Color Encoding Format and Content Color Gamut] while sending
 	 * YCBCR 420, HDR BT.2020 signals we should program MSA MISC1 fields
 	 * which indicate VSC SDP for the Pixel Encoding/Colorimetry Format.
+	 *
+	 * FIXME MST doesn't pass in the conn_state
 	 */
-	if (intel_dp_needs_vsc_sdp(crtc_state, conn_state))
+	if (conn_state && intel_dp_needs_vsc_sdp(crtc_state, conn_state))
 		temp |= DP_MSA_MISC_COLOR_VSC_SDP;
 
 	I915_WRITE(TRANS_MSA_MISC(cpu_transcoder), temp);