tests/kms_atomic: Add test to validate immutable zpos

i915 implements immutable zpos property whereas the existing test
case is written to validate mutable zpos.

Added a new sub-test to validate immutable zpos. Test validates
the reported zpos property of plane by making sure only higher
zpos plane covers the lower zpos one (lower plane covers full screen,
upper plane is 64x64).

Only two planes at a time are tested in increasing fashion to avoid
combinatorial explosion. By transitive property,
if (p1, z1) < (p2, z2) and (p2, z2) < (p3, z3) then (p1, z1) < (p3, z3)

where p and z denotes planes and zpos

v2: -Removed intel only checks [Martin]
    -Used XRGB8888 pixel format as used in other IGTs
    -Added documentation [Martin]
    -Removed skip, instead continue if plane doesn't support zpos [Martin]
    -Sorted planes in increasing order of zpos [Martin]
v3: -Fix description [Martin]
    -Avoid sorting [Ankit]
v4: -Change in commit message [Ankit]
    -Few minor changes [Ankit]
v5: -Use igt_assert_lt() and not plain assert [Petri]
    -Changed upper plane to 64x64 [Ville]
    -Removed no-use break condition [Ville]
    -Enabled cursor test testing too, changed pixel format
     to ARGB8888 from XRGB8888 [Ville]
    -New FBs at every iteration is pointless, removed [Martin, Ville]
v6: -Version 5 brought regression to legacy platforms, changed pixel format
     to XRGB8888 and skipped cursor plane validation as of now

TODO: Enable cursor plane validation

Issue: https://gitlab.freedesktop.org/drm/intel/issues/404


Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>