drm/plane-helper: fix uninitialized variable reference

drm/plane-helper: fix uninitialized variable reference

drivers/gpu/drm/drm_plane_helper.c: In function 'drm_primary_helper_update':
drivers/gpu/drm/drm_plane_helper.c:113:32: error: 'visible' is used uninitialized [-Werror=uninitialized]
  113 |         struct drm_plane_state plane_state = {
      |                                ^~~~~~~~~~~
drivers/gpu/drm/drm_plane_helper.c:178:14: note: 'visible' was declared here
  178 |         bool visible;
      |              ^~~~~~~
cc1: all warnings being treated as errors

visible is an output, not an input. in practice this use might turn out
OK but it's still UB.

Fixes: 6e8229864a2b ("drm/plane-helper: Add drm_plane_helper_check_state()")
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20211007063706.305984-1-alex_y_xu@yahoo.ca