drm/i915/display: Attach content type property for LSPCON

Content type is supported on HDMI sink devices. Attached the
property for the same for LSPCON based devices.

v2: Added the content type programming when we are attaching
the property to connector, as suggested by Ville.

v3: Need to attach content type on intel_dp_add_properties
as creating of new properties is not possible at late_register.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130204738.2443-7-uma.shankar@intel.com