drm/i915: protect access to DP_TP_* on non-dp

DP_TP_{CTL,STATUS} should only be programmed when the encoder is intel_dp.
Checking its current usages intel_disable_ddi_buf() is the only
offender, with other places being protected by checks like
pipe_config->fec_enable that is only set by intel_dp.

v3 (José):
- Using intel_crtc_has_dp_encoder() instead of intel_encoder_is_dp()
(Ville)

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NLucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904213419.27547-4-jose.souza@intel.comSigned-off-by: NJosé Roberto de Souza <jose.souza@intel.com>

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

@@ -3465,10 +3465,12 @@ static void intel_disable_ddi_buf(struct intel_encoder *encoder,
 		wait = true;
 	}
 
+	if (intel_crtc_has_dp_encoder(crtc_state)) {
 		val = I915_READ(DP_TP_CTL(port));
 		val &= ~(DP_TP_CTL_ENABLE | DP_TP_CTL_LINK_TRAIN_MASK);
 		val |= DP_TP_CTL_LINK_TRAIN_PAT1;
 		I915_WRITE(DP_TP_CTL(port), val);
+	}
 
 	/* Disable FEC in DP Sink */
 	intel_ddi_disable_fec_state(encoder, crtc_state);