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

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: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904213419.27547-4-jose.souza@intel.com
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>