drm/i915: Turn intel_digital_port_connected() in a vfunc

drm/i915: Turn intel_digital_port_connected() in a vfunc

Let's get rid of the platform if ladders in
intel_digital_port_connected() and make it a vfunc. Now the if
ladders are at the encoder initialization which makes them a bit
less convoluted.

v2: Add forward decl for intel_encoder in intel_tc.h
v3: Duplicate stuff to avoid exposing platform specific
    functions across files (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311155422.3043-2-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>