Currently, the planner just pops the first available drm plane and if
that can't be used for the DrmHwcLayer it just returns error.

This proposes a slighlty smarter way to do that by trying to see if
any of the DrmPlane can be used for the DrmHwcLayer in question.

More, if the drm_plane doesn't have a fix zorder then we could re-add
him to the list of unused planes, so it could be used for hosting
other less demanding DrmHwcLayers.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>