drivers/gpu/drm/drm_buffer.c: remove unnecessary null test before kfree

This patch removes special case of last element and loops from idx to 0.

Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>