drm/amdgpu/gfx8: reset compute ring wptr on the GPU on resume

Commit 42cdf6f6 ("drm/amdgpu/gfx8: always restore kcq MQDs") made the
ring pointer always to be reset on resume from suspend. This caused compute
rings to fail since the reset was done without also resetting it for the
firmware. Reset wptr on the GPU to avoid a disconnect between the driver
and firmware wptr.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3911


Fixes: 42cdf6f6 ("drm/amdgpu/gfx8: always restore kcq MQDs")
Signed-off-by: Eeli Haapalainen <eeli.haapalainen@protonmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>