drm/i915: force bpp for eDP panels

We've had our fair share of woes already which showed that we can't
rely on the bpc limits in the EDID for eDP panels without risking
black screens. So now we limit the depth by what the BIOS recommends
in the VBT:

commit 2f4f649a
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Mon Nov 12 14:33:44 2012 +0200

    drm/i915: do not ignore eDP bpc settings from vbt

But that's not enough, since at least the panel on my ASUS Zenbook
Prime here is also unhappy if the bpc is too low. Hence just take the
firmware value and dither to get what flimsy panels want.

Like before we ensure that we don't change the bpp if the firmware
doesn't provide a value, see

commit 9a30a61f
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Mon Nov 12 14:33:45 2012 +0200

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

v2: Apparently there are some horribly broken eDP panels around which
only work if the DP link is set up as if we want to driver a 24bpp
mode, but still only work if the data is feed at 18bpp. See

commit 57c21963
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Apr 4 17:19:37 2013 +0200

    drm/i915: revert eDP bpp clamping code changes

for the gory details.

Adjust the patch accordingly and update all the relevant comments.

v3: Give up on the cargo-culting v2 attempt and just enfore the edp
bpp value if it's there. Broken panels be damned!

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>