drm/fb-helper: Add missed unlocks in setcmap_legacy()

setcmap_legacy() does not call drm_modeset_unlock_all() in some exits,
add the missed unlocks with goto to fix it.

Fixes: 964c6006 ("drm/fb-helper: separate the fb_setcmap helper into atomic and legacy paths")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203144248.418281-1-hslester96@gmail.com