display: plugin: Add wrapper functions related to states

display: plugin: Add wrapper functions related to states

struct state has functions, timeout value and name for each state.
Remove direct using of struct states and add wrappers.
This is to change the code used directly into a wrapper form to eliminate
dependence and protect struct states data.

Wrapper in this patch is temporary, so it will be changed later.

Change-Id: I1501dab0f3a61810d1ad7e2ebda1cfb54978944a
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>