drm/i915/dp: Reset cached LTTPR count if number of LTTPRs is unsupported

drm/i915/dp: Reset cached LTTPR count if number of LTTPRs is unsupported

After detection the cached LTTPR count can be checked to determine if
LTTPRs in non-transparent mode were detected. Reset the cached LTTPR
count if the reported number of LTTPRs is invalid to ensure the above
checks work as expected.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708190029.271247-4-imre.deak@intel.com