drm/i915: Move CURSIZE setup to i845_update_cursor()

CURSIZE register exists on 845/865 only, so move it to
i845_update_cursor(). Changes to cursor size must be done only when the
cursor is disabled, so do the write just before enabling the cursor.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>