drm/amd/pm: drop unnecessary table existence and dpm enablement check

Either this was already performed in parent API. Or the table is
confirmed to exist.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>