drm/i915: WARN for eDP encoders in intel_dp_detect_dpcd()

We are not calling this function for eDP, so add an early assert about
this for clarity.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509173446.31095-7-imre.deak@intel.com