drm/amdgpu/sdma6: split out per instance resume function

Extract the resume sequence for individual sdma instance from sdma_v6_0_gfx_resume.
The function could be used for start/restart scenario on a certain instance.

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