drm/i915/dp: Fix potential infinite loop in 128b/132b SST

drm/i915/dp: Fix potential infinite loop in 128b/132b SST

Passing 0 as the step only works when there are other reasons to break
out of the BPP loop in intel_dp_mtp_tu_compute_config(). Otherwise, an
infinite loop might occur. Fix it by explicitly checking for 0 step.

Fixes: ef0a0757bbea ("drm/i915/dp: compute config for 128b/132b SST w/o DSC")
Reported-by: Imre Deak <imre.deak@intel.com>
Closes: https://lore.kernel.org/r/Z6I0knh2Kt5T0JrT@ideak-desk.fi.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204154925.3001781-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>