drm/amd/powerplay/hwmgr: Remove null check before kfree

kfree on NULL pointer is a no-op and therefore checking is redundant.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>