msm: mdss: Enable solid fill on pipe

Add support to enable solid fill on pipe with an option to
configure fill-color by userspace. This support does not confine
to background pipe and can be used to enable solid fill on any source
surface processing pipes irrespective of the z-order.
This support can be used in requirements involving constant
color layers.

Change-Id: Iae3c2968ca2f2f1be161e9c2f857641ffbb859e5
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>