msm: mdss: add support to change border color through debugfs

During blending operation, bottom color is known as border color and it
is same size as mixer dimensions. Default color is black but it can be
configured to different color. This is very useful for debugging purpose.
Add support to program border-color through debugfs as per following
guidelines.

echo code > <debugfs>/mdp/bordercolor/colorN
where
    code: 12bit color value in decimal
    colorN: color0, color1, color2

Change-Id: Ia3e8791713f5aa309a95c867445995ced028dd8d
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>