Revert "drm/amd/display: Implement zpos property"

This reverts commit 5b29369b.

This patch causes IGT test case 'kms_atomic@plane-immutable-zpos' to
fail on AMDGPU hardware.

Cc: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Nicholas Choi <Nicholas.Choi@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>