drm/i915: use pipe bpp in DP link bandwidth calculation

Now that we track bpp on a per-pipe basis, we can use the actual value
rather than assuming 24bpp.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>