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

Documentation/gpu/drm-mm.rst

@@ -449,6 +449,9 @@ PRIME Helper Functions
 PRIME Function References
 -------------------------
 
+.. kernel-doc:: include/drm/drm_prime.h
+   :internal:
+
 .. kernel-doc:: drivers/gpu/drm/drm_prime.c
    :export: