drm/amd/pm: Clean up errors in hwmgr.h

Fix the following errors reported by checkpatch:

ERROR: open brace '{' following struct go on the same line
ERROR: Use C99 flexible arrays

Signed-off-by: Ran Sun <sunran001@208suo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>