drm/amd/display: Remove redundant condition.

You cannot have modeset and flip in the same call for
same CRTC, in such case it will be set mode and set plane,
not a flip.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>