drm/i915: clean up PIPECONF bpc #defines

Ilk+ somehow used #defines in near the PIPESTAT definitions, which
decently confused me. Earlier platforms called it BPP instead of
BPC. Clean this all up.

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