drm/i915/dpio: Rename some variables

Use a consistent 'tmp' as the variable name for the register
values during rmw when we don't deal with multiple registers
in parallel.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422083457.23815-7-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>