drm/amdgpu: fix acpi build warnings

Two newly introduced functions are in the global namespace but have no prototypes
or callers outside of amdgpu_acpi.c, another function is static but only has
a caller inside of an #ifdef:

drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:902:13: error: no previous prototype for 'amdgpu_acpi_get_node_id' [-Werror=missing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:928:30: error: no previous prototype for 'amdgpu_acpi_get_dev' [-Werror=missing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c:860:33: error: 'amdgpu_acpi_get_numa_info' defined but not used [-Werror=unused-function]

Avoid the warnings by marking all of them static and ensuring that the compiler is
able to see the callsites.

v2: rebase on latest code (Alex)

Fixes: fa0497c3 ("drm/amdgpu: Add API to get numa information of XCC")
Fixes: 1cc82301 ("drm/amdgpu: Store additional numa node information")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>