drm/i915/gen9+: Enable hotplug detection early

For LSPCON resume time initialization we need to sample the
corresponding pin's HPD level, but this is only available when HPD
detection is enabled. Currently we enable detection only when enabling
HPD interrupts which is too late, so bring the enabling of detection
earlier.

This is needed by the next patch.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485509961-9010-2-git-send-email-imre.deak@intel.com