drm/amd/pm: fix null ptr access

check null ptr first before access its element

v2: check adev->pm.dpm_enabled early in amdgpu_debugfs_pm_init()

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