drm/amdgpu/sdma4: simplify the logic around powering up sdma

Just check if it's an APU.  The checks for the ppfuncs are
pointless because if we don't have them we can't power up
sdma anyway so we shouldn't even be in this code in the first
place.  I'm not sure about the in_gpu_reset check.  This
probably needs to be double checked.  The fini logic doesn't
match the init logic however with that in_gpu_reset check
in place which seems odd.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>