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/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 866b294..9fb65b7 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -29,8 +29,9 @@
 #include <linux/export.h>
 #include <linux/dma-buf.h>
 #include <linux/rbtree.h>
-#include <drm/drmP.h>
+#include <drm/drm_prime.h>
 #include <drm/drm_gem.h>
+#include <drm/drmP.h>
 
 #include "drm_internal.h"