drm/i915/display: Only query DP state of a DDI encoder

Avoid a NULL dereference for a mismatched encoder type, hit when
probing state for all encoders.

This is a band aid to prevent the OOPS as the right fix is "probably to
swap the psr vs infoframes.enable checks, or outright disappear from
this function" (Ville).

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1892
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525124912.16019-1-chris@chris-wilson.co.uk
(cherry picked from commit 22da5d8)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

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

@@ -5206,6 +5206,9 @@ void intel_read_dp_sdp(struct intel_encoder *encoder,
 		       struct intel_crtc_state *crtc_state,
 		       unsigned int type)
 {
+	if (encoder->type != INTEL_OUTPUT_DDI)
+		return;
+
 	switch (type) {
 	case DP_SDP_VSC:
 		intel_read_dp_vsc_sdp(encoder, crtc_state,