drm/fb-helper: Document struct drm_fb_helper.hint_leak_smem_start

Document the new field smem_start in struct drm_fb_helper and avoid
a compile-time warning. An error message is shown below and the bug
report is at [1].

  include/drm/drm_fb_helper.h:204: warning: Function parameter or member 'hint_leak_smem_start' not described in 'drm_fb_helper'

Reported-by: Stephen Rothwell <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Signed-off-by: Thomas Zimmermann <[email protected]>
Fixes: e7c5c29 ("drm/fb-helper: Set flag in struct drm_fb_helper for leaking physical addresses")
Cc: Thomas Zimmermann <[email protected]>
Cc: Javier Martinez Canillas <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/dri-devel/[email protected]/T/#u # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: tdz

include/drm/drm_fb_helper.h

@@ -200,6 +200,13 @@ struct drm_fb_helper {
 	 */
 	int preferred_bpp;
 
+	/**
+	 * @hint_leak_smem_start:
+	 *
+	 * Hint to the fbdev emulation to store the framebuffer's physical
+	 * address in struct &fb_info.fix.smem_start. If the hint is unset,
+	 * the smem_start field should always be cleared to zero.
+	 */
 	bool hint_leak_smem_start;
 };