drm/i915: allow high-bpc modes on DP

Totally untested due to lack of screens supporting more than 8bpc. But
now we should have closed all holes in our bpp handling, so this
should be safe. The last missing piece was 10bpc support for g4x/vlv,
since we directly use the pipe bpp to feed the display link (and
anyway, only the cpt has any means to have a pipe bpp != the display
link bpp).

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>