drm: Introduce helper for replacing blob properties

Introduce a common helper for the pattern of:
  - allocate new blob property
  - potentially free old blob property
  - replace content of indicative property with new blob ID
  - change member pointer on modeset object

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Squash in fixup from Daniel for the kerneldoc, reported by
0day builder.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>