LCDC: Don't call winX_open if already in correct state

No point attempting to set an lcdc to the same state it already is in.
Important: without this check atv_layer_cnt can both overflow and underflow,
causing the lcdc (and the screen) to shut down when it shouldn't.
Merge pull request #32 from lgeek/pr

RK3188 LCDC: Don't call winX_open if already in requested state