drm/i915: drop WaMbcDriverBootEnable workaround

Turns out the BIOS will do this for us as needed, and if we try to do it
again we risk hangs or other bad behavior.

Note that this seems to break libva on ChromeOS after resumes (but
strangely _not_ after booting up).

This essentially reverts

commit b4ae3f22
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Jun 14 11:04:48 2012 -0700

    drm/i915: load boot context at driver init time

and

commit b3bf0766
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Tue Nov 20 13:27:44 2012 -0200

    drm/i915: implement WaMbcDriverBootEnable on Haswell

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-and-Tested-by: Stéphane Marchesin <marcheu@chromium.org>
[danvet: Add note about impact and regression citation.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>