msm: mdss: Document mdss driver parameters

mdss driver parameters, visible to userland, are documented,
giving out short descriptions of their role.

Change-Id: I34bfe0152c7f7629c18a968a474ab0a3369560fa
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>