drm/amdgpu: remove set but not used variable 'psp_enabled'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/nv.c: In function 'nv_common_early_init':
drivers/gpu/drm/amd/amdgpu/nv.c:471:7: warning:
 variable 'psp_enabled' set but not used [-Wunused-but-set-variable]

It's not used since inroduction in
commit c6b6a421 ("drm/amdgpu: add navi10 common ip block (v3)")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>