OMAPDSS: APPLY: add dss_mgr_simple_check()

Add dss_mgr_simple_check() which is used to check the validity of
certain manager attributes. Only attributes that can be checked
independently, without knowing the display being used, is done here
(thus "simple").

We can use this function in dss_mgr_set_info().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>