drm/i915: do not default to 18 bpp for eDP if missing from VBT

drm/i915: do not default to 18 bpp for eDP if missing from VBT

commit 88044f0c0e396d1029260888a9c4fca4978432e5
Author: Zhenyu Wang <zhenyuw@linux.intel.com>
Date:   Wed Jan 13 11:19:52 2010 +0800

    drm/i915: parse eDP panel color depth from VBT block

originally introduced parsing bpp for eDP from VBT, with a default of 18
bpp if the eDP BIOS data block is not present. Turns out that default seems
to break the Macbook Pro with retina display, as noted in

commit 147136a860e65b48280ee8625bbe0e87247032ce
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Aug 10 11:10:20 2012 +0200

    drm/i915: ignore eDP bpc settings from vbt

Since we can't ignore bpc settings from VBT completely after all, get rid
of the default. Do not clamp eDP to 18 bpp by default if the eDP BDB is
missing from VBT.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Henrik Rydberg <rydberg@euromail.se>
[danvet: paste in the updated commit message from irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>