drm_hwcomposer: Support assigning planes in ValidateDisplay

In order to assign planes to layers in ValidateDisplay, testing
compositing with a DRM atomic modeset test is needed as PresentDisplay
is too late. This means most of PresentDisplay needs to be run from
ValidateDisplay, so refactor PresentDisplay to a common function adding
a test only option.

Signed-off-by: Rob Herring <robh@kernel.org>
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