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")

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
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