drm/i915: Replace some open coded intel_crtc_has_dp_encoder()s

A bunch of places still look for DP encoders manually. Just call
intel_crtc_has_dp_encoder(). Note that many of these places don't
look for EDP or DP_MST, but it's still fine to replace them because
* for audio we don't enable audio on eDP anyway
* the code that lack DP MST check is only for plaforms that
  don't support MST anyway

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466621833-5054-8-git-send-email-ville.syrjala@linux.intel.com