lcd: fix the color testpattern in 16bit mode

The testpattern of the lcd was only working in 8bit mode(2x3 tiles in
different colors). With this patch now 8bit and 16bit is supported.
In 16bit mode there are 2x4 tiles in different colors.
The number of LCD-colors is defined in the include/configs/<boardfile>.h

Signed-off-by: Andreas Neubacher <neubacher.andreas@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>