drm/i915/dp: Factor out intel_dp_get_active_pipes()

Factor out a helper used by a follow up patch to reset an active DP
link.

No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-12-imre.deak@intel.com