drm/i915: Rename global latency_ns variable

We use the variable name latency_ns in both the local lowlevel wm
calculation routines and at the global level. Rename the global value to
reduce shadow warnings and future confusion.

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