drm/xe/dp: Fix non-display builds with DP tunnelling incorrectly enabled

Code for the DP tunnelling functionality in the xe driver can be
built only if the display code is also built, adjust the kconfig
dependency accordingly.

Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Fixes: 73900dce ("drm/xe/dp: Enable DP tunneling")
Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117153843.1312303-1-imre.deak@intel.com