drm/amd/display: Move PRIMARY plane zpos higher

[Why]

Compositors have different ways of assigning surfaces to DRM planes for
render offloading. It may decide between various strategies: overlay,
underlay, or a mix of both (see here for more info:
https://gitlab.freedesktop.org/emersion/libliftoff/-/issues/76)

One way for compositors to implement the underlay strategy is to assign
a higher zpos to the DRM_PRIMARY plane than the DRM_OVERLAY planes,
effectively turning the DRM_OVERLAY plane into an underlay plane.

Today, amdgpu attaches an immutable zpos of 0 to the DRM_PRIMARY plane.
This however, is an arbitrary restriction. DCN pipes are general
purpose, and can be arranged in any z-order. To support compositors
using this allocation scheme, we can set a non-zero immutable zpos for
the PRIMARY, allowing the placement of OVERLAYS (mutable zpos range
0-254) beneath the PRIMARY.

[How]

Assign a zpos = #no of OVERLAY planes to the PRIMARY plane. Then, clean
up any assumptions in the driver of PRIMARY plane having the lowest
zpos.

v2: Fix typo s/decending/descending/

Reviewed-by: Harry Wentland <[email protected]>
Acked-by: Zaeem Mohamed <[email protected]>
Signed-off-by: Leo Li <[email protected]>
Acked-by: Pekka Paalanen <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

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