msm: mdss: Use MDP_SOLID_FILL to enable color on pipe

To enable constant color on a pipe, set MDP_SOLID_FILL in pipe
flag. This will do a solid fill on the pipe with the color
specified in bg_color.

Change-Id: I3201e2fa657a6fc6d0e7f51f8585366afc3d29a3
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>