drm: samsung: fix import of DMA_BUF module namespace

This patch fixes errors similar the following:

  ERROR: modpost: module exynos-drm uses symbol dma_buf_mmap
  from namespace DMA_BUF, but does not import it.

Bug: 215712060
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I3504617b4a6a2589667f1b748d03c6e3894cfb36