drm/amd/display: use mpcc_count to log MPC state

According to [1]:
```
DTN only logs 'pipe_count' instances of MPCC. However in some cases
there are different number of MPCC than DPP (pipe_count).
```

As DTN log still relies on pipe_count to print mpcc state, switch to
mpcc_count in all occurrences.

[1] https://lore.kernel.org/amd-gfx/20240328195047.2843715-39-Roman.Li@amd.com/



Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>