drm: Mark PCI AGP helpers as legacy

DRM's AGP helpers for PCI are only required by legacy drivers. Put them
behind CONFIG_DRM_LEGACY and add the _legacy_ infix.

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/20210507185709.22797-4-tzimmermann@suse.de