drm/i915/xe2hpd: Set maximum DP rate to UHBR13.5

Max supported speed by xe2hpd is UHBR13.5. Limit the max DP source rate
to it.

Bspec: 67066
Signed-off-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Reviewed-by: Shekhar Chauhan <shekhar.chauhan@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430172850.1881525-16-radhakrishna.sripada@intel.com