drm/i915/vga: Assert that VGA register accesses are going to the right GPU

drm/i915/vga: Assert that VGA register accesses are going to the right GPU

We want our VGA register accesses to land on the correct GPU.
Check that the VGA routing is appropriately configured.

For the iGPU this just means the IO decode enable on the GPU, but
for dGPUs we also need the entire chain of bridges to forward the
VGA accesses.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-10-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>