drm/amd/amdgpu: Enable UVD PG on Tonga

Tested by reading tile/clk bits during load/idle.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>