drm/i915: add common intel_digital_port_connected function

Add a common intel_digital_port_connected() that splits out to functions
for different platforms. No functional changes.

v2: make the function return a boolean

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>