drm/amdgpu/{uvd,vcn}: fetch ring's read_ptr after alloc

drm/amdgpu/{uvd,vcn}: fetch ring's read_ptr after alloc

[What]
readptr read always returns zero, since most likely
these blocks are either power or clock gated.

[How]
fetch rptr after amdgpu_ring_alloc() which informs
the power management code that the block is about to be
used and hence the gating is turned off.

Signed-off-by: Louis Li <Ching-shih.Li@amd.com>
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>