compositor-drm: Allow scanout plane to be occluded by overlay

a0f8276f ("compositor-drm: Disallow overlapping overlay planes") was
a little too pessimistic in rejecting occluded views. Whilst it
correctly prevented overlay planes from occluding each other, it also
prevented overlay planes from occluding the scanout plane.

This is undesirable: the primary/scanout plane is specified to stack
strictly below all overlay planes, so there is no need to reject a plane
from consideration for scanout due to being occluded by an overlay
plane.

Shift the check downwards so it only applies to overlay rather than
scanout planes.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>