drm_hwcomposer: Set zpos relative to the minimum possible value

Current implementation doesn't handle properly the cases where zpos
range starts from 1.
See drm-hwcomposer/drm-hwcomposer#19 (comment 100622)

Fixes: ea1c5e5a ("drm_hwcomposer: Add z order support")

Signed-off-by: Alexandru Gheorghe <alexc.g1.ro@gmail.com>
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