drm/amd/amdgpu: Indent AMD_IS_APU properly

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/soc15.c:715 soc15_get_pcie_usage() warn: inconsistent indenting
And a similar one in si.c.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>