drm_hwcomposer: Add z order support

Alexandru Gheorghe requested to merge alex_g1/drm-hwcomposer:z_order into master

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

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