drm/amdgpu/sdma5: split out per instance resume function

Extract the resume sequence from sdma_v5_0_gfx_resume for
starting/restarting an individual instance.

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