drm/radeon: Move AGP helpers into radeon driver

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