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

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

This reverts commit 5d21ab0b8e115968de8286336a7c8b680c6ab295.

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>