msm: mdss: declare function used only within the file as static

Fix sparse tool error which mandates that functions used only
within the file to be declared as static to allow compiler
optimizations.

Change-Id: Iba14c50730bb7ce402e4b45bc9b2c8d1fec6f3eb
Signed-off-by: Ashish Garg <ashigarg@codeaurora.org>