drm: kill the ->agp_destroy callback

Call drm_pci_agp_destroy directly, there's no point in the
indirection. Long term we want to shuffle this into each driver's
unload logic, but that needs cleared-up drm lifetime rules first.

v2: Add a dummy function for !CONFIG_PCI, spotted my David Herrmann.

v3: Fixup for the coding style police.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>