display: Add display hal-backend check function

display: Add display hal-backend check function

Before plugin to core centralization, display hal backend check is needed.
To move functions without plugin dependency, this temporary function is necessary.
With this, plugin functions can be moved to core(deviced/src/display module).
These functions will be modified during plugin to core centralization.

void display_set_hal_backend_available(bool display_check_hal_backend)
-> With this, while plugin probing, deviced core can know whether hal_backend is
	available or not.
bool display_check_hal_backend_available(void)
-> This function is for replacing the display_dev_available variable of plugins.

Change-Id: I1e926da29ab097e08533f73136a2f928daca9167
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>