drm: Unexport primary plane helpers

Well except the destroy helper, which isn't really a primary helper
but generally useful, if mislabelled.

v2: Keep some of the nice comments about the limitations of the
primarmy plane helpers, and put them into the kerneldoc for
drm_crtc_init() (Sam).

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005094732.31353-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_modeset_helper.c

@@ -146,6 +146,21 @@ static struct drm_plane *create_primary_plane(struct drm_device *dev)
  * Initialize a CRTC object with a default helper-provided primary plane and no
  * cursor plane.
  *
+ * Note that we make some assumptions about hardware limitations that may not be
+ * true for all hardware:
+ *
+ * 1. Primary plane cannot be repositioned.
+ * 2. Primary plane cannot be scaled.
+ * 3. Primary plane must cover the entire CRTC.
+ * 4. Subpixel positioning is not supported.
+ * 5. The primary plane must always be on if the CRTC is enabled.
+ *
+ * This is purely a backwards compatibility helper for old drivers. Drivers
+ * should instead implement their own primary plane. Atomic drivers must do so.
+ * Drivers with the above hardware restriction can look into using &struct
+ * drm_simple_display_pipe, which encapsulates the above limitations into a nice
+ * interface.
+ *
  * Returns:
  * Zero on success, error code on failure.
  */