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>
[seanpaul converted to std::tuple return type]
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Change-Id: I35dc2c1cfd0e38ca3a47cf4e668eeb5f3c470ddb