drm/atomic: Hide drm.ko internal interfaces

This is just a bit fallout from patch polishing and moving the
get_prop logic fully into the core:
- Drop EXPORT_SYMBOL and make the helpers static.
- Drop kerneldoc since not used by drivers.
- Move the cross-file function declarations only used by drm.ko
  internally to an internal header.

v2: keep the gist of the comments, requested by Rob.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_ioctl.c

@@ -32,6 +32,7 @@
 #include <drm/drm_core.h>
 #include "drm_legacy.h"
 #include "drm_internal.h"
+#include "drm_crtc_internal.h"
 
 #include <linux/pci.h>
 #include <linux/export.h>