Revert "drm/i915: revert eDP bpp clamping code changes"

This reverts commit 57c21963.

It's an ugly hack for a Haswell SDV platform where the vbt doesn't
seem to fully agree with the panel. Since it seems to cause issues on
real eDP platform let's just kill this hack again.

Reported-and-tested-by: Josh Boyer <jwboyer@gmail.com>
References: https://lkml.org/lkml/2013/5/3/467


Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>