i915: Update VGA arbiter support for newer devices

This is intended to add VGA arbiter support for Intel HD graphics on
Core processors.  The old GMCH registers no longer exist, so even
though it appears that i915 participates in VGA arbitration, it doesn't
work.  On Intel HD graphics we already attempt to disable VGA regions
of the device.  This makes registering as a VGA client unnecessary since
we don't intend to operate differently depending on how many VGA devices
are present.  We can disable VGA memory regions by clearing the memory
enable bit in the VGA MSR.  That only leaves VGA IO, which we update
the VGA arbiter to know that we don't participate in VGA memory
arbitration.  We also add a hook on unload to re-enable memory and
reinstate VGA memory arbitration.

v3: Use explicit LEGACY_IO | LEGACY_MEM when restoring rather than
    LEGACY_MASK, per Ville's comments.

v2: I915_READ/WRITE accessors don't work in i915_disable_vga, use inb/outb
    directly.  Also, on the driver unbind VGA enable path,...