drm/i915: use pipe bpp when setting HDMI bpc

The Intel HDMI encoder can support 8bpc or 12bpc.  Set the appropriate
value based on the pipe bpp when configuring the output.

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>