gma500_gfx: Disable backlight initialization

Backlight initialization doesn't work on the Joggler and causes the loading of the gm500_gfx module to abort.
gma500_gfx: Force use of 16 bit framebuffer

Android needs a 16 bit framebuffer for fast OpenGL emulation.
[gma500_gfx] Disable broken GTT acceleration

The use of GTT acceleration on the Joggler leads to a corrupted screen display.
[gma500_gfx] Debugging Stuff