x86: add early quirk for reserving Intel graphics stolen memory v5

Systems with Intel graphics controllers set aside memory exclusively for
gfx driver use.  This memory is not always marked in the E820 as
reserved or as RAM, and so is subject to overlap from E820 manipulation
later in the boot process.  On some systems, MMIO space is allocated on
top, despite the efforts of the "RAM buffer" approach, which simply
rounds memory boundaries up to 64M to try to catch space that may decode
as RAM and so is not suitable for MMIO.

v2: use read_pci_config for 32 bit reads instead of adding a new one
    (Chris)
    add gen6 stolen size function (Chris)
v3: use a function pointer (Chris)
    drop gen2 bits (Daniel)
v4: call e820_sanitize_map after adding the region
v5: fixup comments (Peter)
    simplify loop (Chris)

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66726
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66844
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>