drm_hwcomposer: Fix ValidateDisplay() when lowest z-order is nonzero

ValidateDisplay()'s algorithm for achieving minimal GPU load assumes
that the lowest z-order is zero and that layers have sequential z-orders.
CalcPixOps() and MarkValidated() are also written with the same assumption.
However, there is no such guarantee provided by SurfaceFlinger and VTS
tests like PRESENT_DISPLAY_NO_LAYER_STATE_CHANGES fail as they only have
one layer with z-order of 10.

Normalise the mapping between layers and z-order so that the algorithm works
as intended.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Fixes commit b7b81cfb ("drm_hwcomposer: Choose client layer range to
achieve minimal GPU load")