drm/i915: Make intel_digital_port_connected() work for any port

Add the missing port A handling to intel_digital_port_connected()
and also separate SPT from the CPT/LPT code a bit.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170615171252.11921-1-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>