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

This reverts commit e2066eb4.

The problematic IGT test case (i.e. kms_atomic@plane-immutable-zpos) has
been fixed as of commit cb77add45011 ("tests/kms_atomic: remove zpos <
N-planes assert") to the IGT repo. So, reintroduce the reverted code.

Link: drm/igt-gpu-tools@cb77add4


Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>