drm/i915/vga: Stop trying to use GMCH_CTRL for VGA decode control

intel_gmch_vga_set_state() is a complete lie on ILK+ because
the GMCH_CTRL register is locked and can't actually be written.
But we still need to remove the iGPU from the VGA arbitration
on iGPU+dGPU systems, or else Xorg performance will tank due
to the constant VGA arbiter accesses.

For VGA memory decode we can't turn off the PCI_COMMAND
memory deocde as that would disable even normal MMIO.
Instead we can disable just the VGA memory decode via
the VGA MSR register. And we can do that just once
when disabling the VGA plane. That way we don't have
to touch VGA registers anywhere else.

We can also inform the arbiter that we're no longer decoding
VGA memory. This will stop the arbiter from disabling all
memory decode for the iGPU via PCI_COMMAND (and thus breaking
everything) whenever some other GPU wants to own the VGA memory
accesses.

For IO we can disable all IO decode via the PCI_COMMAND
register, except around the few VGA register accesses that
we need to do in intel_vga_disable(). Unfortunately we can't
disable IO decode permanently as it makes some laptops (eg.
Dell Latitude E5400) hang during reboot/shutdown. One option
would be to re-enable IO decode from the poweroff hooks, but
that won't help the sysrq emergency reboot/shutdown since it
won't call said hooks. So let's try to keep IO decode in its
original setting unless we really need to disable it to
exclude the GPU from VGA arbitration.

I suppose we could keep frobbing GMCH_CTRL on pre-ILK, but
it seems better to not do it since it has other side effects
such as changing the class code of the PCI device.

For discrete GPUs we'll rely on the bridge control instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-9-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

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