drm/amd/display: Fix double cursor on non-video RGB MPO

[Why]

DC makes use of layer_index (zpos) when picking the HW plane to enable
HW cursor on. However, some compositors will not attach zpos information
to each DRM plane. Consequently, in amdgpu, we default layer_index to 0
and do not update it.

This causes said DC logic to enable HW cursor on all planes of the same
layer_index, which manifests as a double cursor issue if one of the
planes is scaled (and hence scaling the cursor as well).

[How]

Use DRM core helpers to calculate a normalized_zpos value for each
drm_plane_state under each crtc, within the atomic state.

This helper will first consider existing zpos values, and if
identical/unset, fallback to plane ID ordering.

The normalized_zpos is then passed to dc_plane_info during atomic check
for later use by the cursor logic.

Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>