Revert "drm/i915: Calculate correct stolen size for GEN7+"

Revert "drm/i915: Calculate correct stolen size for GEN7+"

This reverts commit 03752f5b7b77b95d83479885040950fba1250850.

This revert requires a bit of explanation on how I understand things
work. Internally the architects/designers decide how the stolen encoding
works. We put it in a doc. BIOS writers take these docs and implement
it. Driver writers read the doc too, and read the value left by the BIOS
writers, and then we make magic.

The failing here is that in the docs we had[1] contained two different
definitions for this register for Gen7. (We have both a PCI register,
and an MMIO, and each of these were different). At the time [2] of
03752f5, we asked the architects what the correct value should be; but
that doesn't match the reality (BIOS) unfortunately.

So on all machines I can get my hands on, this revert is the right thing
to do. I've also worked with the product group to confirm that they
agree this revert is what we should do. People using HW made my "people"
who both write their own BIOS, and have access to our docs (Apple?).
Investigations are still ongoing about whether we need to add a list
of machines needing special handling, but this patch should be the
right thing for pretty much everyone.

[1] The docs are still wrong on this one. Now instead of two registers with
two definitions, we have one register with BOTH definitions, progress?
[2] The open source PRMs have the "wrong" definitions in chapter Volume
1 part6, section 1.1.12.

This digging was inspired by Paulo.

Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Augment the patch saying that it's still a bit unclear
whether there are any machines out there with "wrong" firmware and
whether we need to add a list to handle them specially.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>