drm/i915: Constify the drm_i915_private pointer a bit more

A lot of the WM functions are only reading from that structure and are
already using const. While converting the code to use dev_priv instead
of dev, I noticed a few places where we can give that hint.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>