drm: prime: Document gem_prime_mmap

gem_prime_map is not currently described in the DRM manual, lets document
it.
Signed-off-by: NDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_prime.c

@@ -309,7 +309,7 @@ static const struct dma_buf_ops drm_gem_prime_dmabuf_ops =  {
  * Drivers can implement @gem_prime_export and @gem_prime_import in terms of
  * simpler APIs by using the helper functions @drm_gem_prime_export and
  * @drm_gem_prime_import.  These functions implement dma-buf support in terms of
- * five lower-level driver callbacks:
+ * six lower-level driver callbacks:
  *
  * Export callbacks:
  *
@@ -321,6 +321,8 @@ static const struct dma_buf_ops drm_gem_prime_dmabuf_ops =  {
  *
  *  - @gem_prime_vunmap: vunmap a buffer exported by your driver
  *
+ *  - @gem_prime_mmap (optional): mmap a buffer exported by your driver
+ *
  * Import callback:
  *
  *  - @gem_prime_import_sg_table (import): produce a GEM object from another