drm/i915: Shuffle sprite register writes into a tighter group

Group the sprite register writes a bit tighter. We want to write
the registers atomically, and so doing the base address/offset
artihmetic within the critical section is pointless when it can
all be done beforehand.

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