drm/amdgpu/vcn2.0: convert internal functions to use vcn_inst

drm/amdgpu/vcn2.0: convert internal functions to use vcn_inst

Pass the vcn instance structure to these functions rather
than adev and the instance number.

TODO: clean up the function internals to use the vinst state
directly rather than accessing it indirectly via adev->vcn.inst[].

v2: index instances directly on vcn1.0 and 2.0 to make
it clear that they only support a single instance (Lijo)

Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>