drm/i915: correctly set the DDI_FUNC_CTL bpc field

Correctly erase the values previously set and also check for 6bpc and
10bpc.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>