vt: might_sleep() annotation for do_blank_screen

For symmetry reasons with do_unblank_screen, except without the
oops_in_progress special case.

Just a drive-by annotation while I'm trying to untangle the fbcon vs.
fbdev screen blank/unblank maze.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Adam Borowski <kilobyte@angband.pl>
Cc: Martin Hostettler <textshell@uchuujin.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-4-daniel.vetter@ffwll.ch