OMAPDSS: panel acx565akm: remove omap_dss_device maximum backlight level usage

The omap_dss_device structs's max_backlight_level is used to pass maximum
backlight level for the platform. However, no board file using this panel
populates this field. Therefore, we remove it's usage from the panel driver.

Signed-off-by: Archit Taneja <archit@ti.com>