drm/i915/dp: remove accidental static on what should be a local variable

The variable should obviously be local, not static.

Fixes: a421d8a9

 ("drm/i915/dp: rewrite DP 2.0 128b/132b link training based on errata")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217083938.3587465-1-jani.nikula@intel.com