drm/i915/dp_mst: Fix disabling the minimum HBlank time

drm/i915/dp_mst: Fix disabling the minimum HBlank time

Disable the minimum HBlank time only on LNL+, where this functionality
and corresponding register exists.

Bspec: 74379
Fixes: a5ebe00c2ace ("drm/i915/dp: Guarantee a minimum HBlank time")
Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Suraj Kandpal <suraj.kandpal@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/20250206164624.3185280-1-imre.deak@intel.com