GUI: Set brightness after gr_init()

On at least one device I've tested (htc msm8960), setting the
brightness after gr_init() is necessary for the splash to show.

Note that at this point in the boot process, the settings file has
not yet been read, so tw_brightness has its default value of max
brightness. When settings are finally loaded, the correct brightness
is loaded for the GUI.

Change-Id: Ifc58c1dc80ed9e6f6b2a269a9bb6764038626c84