drm/i915/tc: Factor out a function querying active links on a TC port

For clarity factor out the function to determine if there are active
links on a TC port. This prepares for the next patch also checking the
port's PLL type.

While at it pass crtc_state to intel_tc_port_sanitize_mode(), and check
hw.active in that, instead of the deprecated crtc->active flag.

v2: Check crtc_state->hw.active instead of crtc->active. (Ville)

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/20230321220101.983366-2-imre.deak@intel.com