drm/amdgpu: drop pm_sysfs_en flag from amdgpu_device structure

pm_sysfs_en is overlapped with pm.sysfs_initialized, so drop it
for simplifying code(no functional change).

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>