drm/i915/dp: Kill intel_dp->detect_done flag

The intel_dp->detect_done flag is no more useful. Pull
intel_dp_long_pulse() into the lone caller,

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180927205735.16651-5-dhinakaran.pandiyan@intel.com