Radeon calls DRMs core AGP helpers. These helpers are only required
by legacy drivers. Reimplement the code in radeon to uncouple radeon
from the legacy code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507185709.22797-2-tzimmermann@suse.de