drm/amdgpu: Revert programming of CP_PSP_XCP_CTL

Programming of this register is taken care by PSP. Incorrect programming
causes CP not to detect its XCC.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reported-by: Alexander Turek <Alexander.Turek@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>