drm/amd/amdgpu: make sure VCE is disabled by default

With the current code, when we boot with the amdgpu
driver enabled and loaded, the VCE also automatically
remains enabled since bootup. This can be verified from
the output of amdgpu_pm_info. It does not matter whether
we boot into command line directly or into X, the VCE
stays enabled the entire time.

This patch addresses the issue and makes sure that
VCE is turned on only during playback, and remains
disaled otherwise.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>