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

The pipe may be driving various bpp values depending on the display
configuration, so take that into account when calculating link bandwidth
requirements.

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>