drm/i915/vga: Register vgaarb client later

drm/i915/vga: Register vgaarb client later

Currently we register to vgaarb way too early. Thus it is
possible that the entire VGA decode logic gets nuked via
GMCH_CTRL before intel_vga_disable() has even disabled the
VGA plane. This could even cause a system hang because
we'll be unable to turn off the VGA plane gracefully.

Move the vgaarb registration into intel_display_driver_register().
I suppose we could do it a bit earlier (after intel_vga_disable()),
but not convinced there's any point.

Also the error handling here is pointless since the
registration can't fail (unless the device isn't a VGA class
in which case all VGA decode logic should aleady be disabled
by the BIOS via GMCH_CTRL). But let's toss in a WARN to catch
any future breakage of vga_client_register().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>