Fix the code that sets up the MCH BAR on systems where the (buggy) BIOS

Fix the code that sets up the MCH BAR on systems where the (buggy) BIOS
doesn't do this for us.  The code was poking registers on the wrong PCI
device.  We were just lucky that it worked on most systems.

This should fix machines such as the Asus EeePC 701 and get rid of the

error: [drm:pid0:i915_gem_detect_bit_6_swizzle] *ERROR* Couldn't read from
MC HBAR.  Disabling tiling.

messages on that machine.