drm/i915: Nuke intel_dp->channel_eq_status

intel_dp->channel_eq_status is used in exactly one function, and we
don't need it to persist between calls. So just go back to using a
local variable instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180117192149.17760-4-ville.syrjala@linux.intel.com