drm: inline drm_agp_destroy

Wrapping a kfree is pointless.

v2: Add a comment to the kerneldoc for drm_agp_init to explain where
the kfree happens as requested by David. Note that for modeset drivers
agp cleanup is fairly complicated anyway: The drm_agp_clear is a noop
and drivers must call drm_agp_release on their own. Which they all
seem to do properly.

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