intel/i945/gma: Place GTT below top of memory

Since commit 17fec8a0 [1]

	drm/i915: Use Graphics Base of Stolen Memory on all gen3+

present in the Linux kernel since version 3.12, 3D does not work
anymore [2].

Comparing the graphics registers, in this case that means output of
`intel_reg_dumper`, the vendor Video BIOS is setting the register
PGTBL_CTL/PGETBL_CTL, only documented in the i965 datasheet [3], to
`0x3ffc0001` on a system with 1 GB of RAM, while native graphics init
sets it to `0x3f800001`.

Currently native graphis init sets the GTT right above the base
address of stolen memory. The Video BIOS sets it below the top of
memory.  The Linux Intel driver expects it to be below top of memory, so
do it this way, by setting the address to TOM minus the size of the GTT,
which is hardcoded to 256 KiB.

As `PGETBL_CTL` is zero by default, reading its value in the beginning
is not necessary and is only confusing. Make it clear that the code
calculates the value.

There is still a PTE error reported during boot, but 3D works
with Linux 3.12+ and no user visible problems are shown.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=17fec8a08698bcab98788e1e89f5b8e7502ababd
[2] https://bugs.freedesktop.org/show_bug.cgi?id=79038
[3] https://01.org/linuxgraphics/sites/default/files/documentation/965_g35_vol_1_graphics_core_0.pdf
    Intel ® 965 Express Chipset Family and
      Intel ® G35 Express Chipset Graphics Controller
      Programmer’s Reference Manual
    Volume 1: Graphics Core
    Revision 1.0a

Change-Id: I0a5b04c2c5300f5056cb48075aa5804984bc9948
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Francis Rowe <info@gluglug.org.uk>
Reviewed-on: http://review.coreboot.org/5927
Tested-by: build bot (Jenkins)
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>