drm/amd/powerplay: fix various dereferences of a pointer before it is null checked

There are several occurrances of the pointer hwmgr being dereferenced
before it is null checked.  Fix these by performing the dereference
of hwmgr after it has been null checked.

Addresses-Coverity: ("Dereference before null check")
Fixes: c9ffa427

 ("drm/amd/powerplay: enable pp one vf mode for vega10")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>