drm/amdgpu: fix ring test failure issue during s3 in vce 3.0 (V2)

[What]
vce ring test fails consistently during resume in s3 cycle, due to
mismatch read & write pointers.
On debug/analysis its found that rptr to be compared is not being
correctly updated/read, which leads to this failure.
Below is the failure signature:
	[drm:amdgpu_vce_ring_test_ring] *ERROR* amdgpu: ring 12 test failed
	[drm:amdgpu_device_ip_resume_phase2] *ERROR* resume of IP block <vce_v3_0> failed -110
	[drm:amdgpu_device_resume] *ERROR* amdgpu_device_ip_resume failed (-110).

[How]
fetch rptr appropriately, meaning move its read location further down
in the code flow.
With this patch applied the s3 failure is no more seen for >5k s3 cycles,
which otherwise is pretty consistent.

V2: remove reduntant fetch of rptr

Signed-off-by: Louis Li <Ching-shih.Li@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org