drm/i915: utilize subconnector property for DP

Since DP-specific information is stored in driver's structures, every
driver needs to implement subconnector property by itself.

v2: updates to match previous commit changes

v3: rebase

v4: renamed a function call

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Jeevan B <jeevan.b@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> #and acked for merging
Tested-by: Oleg Vasilev <oleg.vasilev@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1587732655-17544-2-git-send-email-jeevan.b@intel.com