drm/i915: Throw out some useless variables

Drop some useless 'reg' variables when we only use them once.

v2: A few more, including a few variable moves

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>