backend-drm: Select plane based on current attached CRTC

When doing plane selection for an output CRTC check if the plane
already has a CRTC attached and if so prefer that plane only for
the corresponding CRTC.

This prevents changing a CRTC's primary plane when it is active
which is not allowed by the DRM framework.

Based-on-patch-by: Eric Ruei <e-ruei1@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>