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

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

This reverts commit e0579b5b3430339d0e0254530d9c49787393de02.

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>