drm: Make drm_pci_agp_init legacy

Pull the drm_pci_agp_init() underneath the legacy ifdeffry alongside its
only caller.

The diff chooses it to so it by moving drm_pci_agp_destroy earlier, but
the important bit is moving the #ifdef earlier before drm_pci_agp_init.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200307093702.2269-1-chris@chris-wilson.co.uk