drm/i915/dp: don't mention eDP bpp clamping if it doesn't affect bpp

This is useful with the follow-up patch that frobs
dev_priv->vbt.edp_bpp, and the value no longer comes directly from
VBT.

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