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

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

This reverts commit 57c219633275c7e7413f8bc7be250dc092887458.

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>