drm/msm/mdp5: handle non-fullscreen base plane case

If the bottom-most layer is not fullscreen, we need to use the BASE
mixer stage for solid fill (ie. MDP5_CTL_BLEND_OP_FLAG_BORDER_OUT).  The
blend_setup() code pretty much handled this already, we just had to
figure this out in _atomic_check() and assign the stages appropriately.

Also fix the case where there are zero enabled planes, where we also
need to enable BORDER_OUT.

Signed-off-by: Rob Clark <robdclark@gmail.com>