fbdev: Rework fb_blank()

Reimplement fb_blank() to return early on errors. No functional
changes. Prepares the helper for tracking the blanking state in
struct fb_info.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250321095517.313713-2-tzimmermann@suse.de


Signed-off-by: Lee Jones <lee@kernel.org>