drm: Mark AGP implementation and ioctls as legacy

Only UMs drivers use DRM's core AGP code and ioctls. Mark the icotls
as legacy. Add the _legacy_ infix to all AGP functions. Move the
declarations to the public and internal legacy header files. The agp
field in struct drm_device is now located in the structure's legacy
section. Adapt drivers to the changes.

AGP code now depends on CONFIG_DRM_LEGACY.

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/20210507185709.22797-5-tzimmermann@suse.de