x86/Sandy Bridge: reserve pages when integrated graphics is present

SNB graphics devices have a bug that prevent them from accessing certain
memory ranges, namely anything below 1M and in the pages listed in the
table.  So reserve those at boot if set detect a SNB gfx device on the
CPU to avoid GPU hangs.

Stephane Marchesin had a similar patch to the page allocator awhile
back, but rather than reserving pages up front, it leaked them at
allocation time.

[ hpa: made a number of stylistic changes, marked arrays as static
  const, and made less verbose; use "memblock=debug" for full
  verbosity. ]

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>