drm/vc4: Release workaround buffer and DMA in error paths and unbind

On Pi0-3 the driver allocates a buffer and requests a DMA channel
because the ARM can't write to DSI1's registers directly.
However unbind and the error paths in bind don't release the buffer or
the DMA channel.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>