msm: mdss_fb: update max brightness set for led backlight

Led backlight structure allows setting max backlight so that max
backlight is properly clamped by led driver.

Change-Id: Icbc87d9575679c1626c03b0fa5c359ebf4d7e43d
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>