drm/amd/pm: only poweron/off vcn/jpeg when they are valid.

If vcn is disabled in kernel parameters, don't touch vcn,
otherwise it may cause vcn hang.

v2: delete unnecessary logs
v3: move "is_vcn_enabled" check to smu_dpm_setvcn/jpeg_enable (Evan)

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>