drm/amdgpu: use begin/end_use for VCE power/clock gating

This fixes turning power and clock on when it is actually needed.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>