drm/amdgpu: Fix APU handling in amdgpu_pm_load_smu_firmware()

We only need to skip this on modern APUs.  It's required
on older APUs as it's where start_smu gets called from.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3502
Fixes: 064d9243

 ("drm/amd/pm: avoid to load smu firmware for APUs")
Reviewed-by: Tim Huang <tim.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Tim Huang <Tim.Huang@amd.com>
(cherry picked from commit 608d886c)