drm/via: clean up reclaim_buffers

A few things
- kill reclaim_buffers, it's never ever called because via does not set
  DRIVER_HAVE_DMA
- inline the idlelock dance into the buffer reclaim logic and make it
  a simple preclose cleanup function
- directly call the the dma_quiescent function and kill the needless
  if check.

v2: Actually drop the idlelock when we take it. Reported by James
Simmons.

v3: Rebased onto latest drm-next.

v4: Fixup the refactor.

v5: More fixup the refactor - I've accidentally changed the check for
any master to checking whether the closing fd is the master.

v6: Don't forget to drop the idlelock in the early return path, too.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>