drm/i915: Stop using a 'reg' variable

'reg' is a very non-descriptive name. Just get rid of the silly
local variable and spell out the full register name always.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231101114212.9345-3-ville.syrjala@linux.intel.com


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