disp: msm: sde: disable border color on empty blendstage

This change disables the border color on the layer mixer,
based on the caller's request. This is required to totally
disconnect the layer mixer hardware when it is not
participating in blending the pixels. Having empty blendstage
but border color enabled, allows Layer mixer hw to produce
border pixels even when blend stage is empty.

Change-Id: I8e84aeedffbd42ad793a167a6cc5a3a653864c1a
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>