drm/i915: Always set the DP transcoder config to 8BPC.

The pipe is always set to 8BPC, but here we were leaving whatever
previous bits were set by the BIOS in place.

Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org