drm/i915/dp: remove static variable for aux last status

Add aux_busy_last_status to intel_dp. Don't bother with initializing to
all ones; the only difference is potentially missing logging for one
error case if the readout is all zeros.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002144138.7917-1-jani.nikula@intel.com