drm: Extract drm_prime.h

Plus a little bit more documentation.

v2: Untangle the missing forward decls to make drm_prime|gem.h
free-standing.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-3-daniel.vetter@ffwll.ch
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 3b2a28f..b9ade75 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -34,6 +34,10 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/kref.h>
+
+#include <drm/drm_vma_manager.h>
+
 /**
  * struct drm_gem_object - GEM buffer object
  *