drm/amdgpu/vpe: power on vpe when hw_init

To fix mode2 reset failure.
Should power on VPE when hw_init.

Signed-off-by: Peyton Lee <peytolee@amd.com>
Reviewed-by: Lang Yu <lang.yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>