Fix drmmode_crtc_scanout_create logic

If crtc scanout create successfully, the function of drmmode_crtc_scanout_create should return TURE. This will fix the regression caused by commit: "Make drmmode_crtc_scanout_create/destroy static" (442efe73), as it will result to some function (such as drmmode_set_scanout_pixmap) go to wrong code path and result to NULL pointer.

Fixes: 442efe73 ("Make drmmode_crtc_scanout_create/destroy static")