drm/amd/display: Do not reset planes based on crtc zpos_changed

[Why]

drm_normalize_zpos will set the crtc_state->zpos_changed to 1 if any of
it's assigned planes changes zpos, or is removed/added from it.

To have amdgpu_dm request a plane reset on this is too broad. For
example, if only the cursor plane was moved from one crtc to another,
the crtc's zpos_changed will be set to true. But that does not mean that
the underlying primary plane requires a reset.

[How]

Narrow it down so that only the plane that has a change in zpos will
require a reset.

As a future TODO, we can further optimize this by only requiring a reset
on z-order change. Z-order is different from z-pos, since a zpos change
doesn't necessarily mean the z-ordering changed, and DC should only
require a reset if the z-ordering changed.

For example, the following zpos update does not change z-ordering:

    Plane A: zpos 2 -> 3
    Plane B: zpos 1 -> 2
    => Plane A is still on top of plane B: no reset needed

Whereas this one does change z-ordering:

    Plane A: zpos 2 -> 1
    Plane B: zpos 1 -> 2
    => Plane A changed from on top, to below plane B: reset needed

Fixes: 38e0c3df ("drm/amd/display: Move PRIMARY plane zpos higher")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3569


Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>