fbdev: imx: Constify lcd_ops

'struct lcd_ops' is not modified by core backlight code, so it can be
made const for increased code safety.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20240424-video-backlight-lcd-ops-v2-17-1aaa82b07bc6@kernel.org


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