drm/panel: Fix kernel-doc typo for `follower_lock`

drm/panel: Fix kernel-doc typo for `follower_lock`

In the kernel doc for the `follower_lock` member of `struct drm_panel`
there was a typo where it was called `followers_lock`. This resulted
in a warning when making "htmldocs":

  ./include/drm/drm_panel.h:270: warning:
  Function parameter or member 'follower_lock' not described in 'drm_panel'

Fix the typo.

Fixes: de0874165b83 ("drm/panel: Add a way for other devices to follow panel state")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20230802142136.0f67b762@canb.auug.org.au
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230802074727.1.I4036706ad5e7f45e80d41b777164258e52079cd8@changeid