drm/amdgpu: fix build error without x86 kconfig (v2)

This patch is to fix below build error while we are using the kconfig
without x86.

drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c: In function
'vangogh_get_smu_metrics_data':
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c:300:10:
error: 'boot_cpu_data' undeclared (first use in this function); did you
mean 'boot_cpuid'?
  300 |          boot_cpu_data.x86_max_cores * sizeof(uint16_t));
      |          ^~~~~~~~~~~~~
      |          boot_cpuid
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c: In function
'vangogh_read_sensor':
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c:1320:11:
error: 'boot_cpu_data' undeclared (first use in this function); did you
mean 'boot_cpuid'?
 1320 |   *size = boot_cpu_data.x86_max_cores * sizeof(uint16_t);
      |           ^~~~~~~~~~~~~
      |           boot_cpuid
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c: In function
'vangogh_od_edit_dpm_table':
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c:1460:19:
error: 'boot_cpu_data' undeclared (first use in this function); did you
mean 'boot_cpuid'?
 1460 |   if (input[0] >= boot_cpu_data.x86_max_cores) {
      |                   ^~~~~~~~~~~~~
      |                   boot_cpuid

v2: fix #ifdef and add comment for APU only

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>