drm/amdgpu: Drop DRIVER_USE_AGP

This doesn't do anything except auto-init drm_agp support when you
call drm_get_pci_dev(). Which amdgpu stopped doing with

commit b58c1131
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Fri Jun 2 17:16:31 2017 -0400

    drm/amdgpu: drop deprecated drm_get_pci_dev and drm_put_dev

No idea whether this was intentional or accidental breakage, but I
guess anyone who manages to boot a this modern gpu behind an agp
bridge deserves a price. A price I never expect anyone to ever collect
:-)

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Xiaojie Yuan <xiaojie.yuan@amd.com>
Cc: Evan Quan <evan.quan@amd.com>
Cc: "Tianci.Yin" <tianci.yin@amd.com>
Cc: "Marek Olšák" <marek.olsak@amd.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org