drm: Use drm_mm_insert_node_in_range_generic() for everyone

Remove a superfluous helper as drm_mm_insert_node is equivalent to
insert_node_in_range with a range of [0, U64_MAX].

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-37-chris@chris-wilson.co.uk