drm/msm/mdp4: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/msm/mdp/mdp4/mdp4_lcdc_encoder.c:96:23: warning: no previous prototype for 'get_connector' [-Wmissing-prototypes]
drivers/gpu/drm/msm/mdp/mdp4/mdp4_plane.c:84:5: warning: no previous prototype for 'mdp4_plane_set_property' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>