drm/i915: Remove unused tmp assignment.

These are left overs from the conversion towards intel_de_rmw.

Fixes: aa80b2b1 ("drm/i915/display/panel: use intel_de_rmw if possible in panel related code")
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230224153707.813953-1-rodrigo.vivi@intel.com