drm/amdgpu/gfx9: add emit_reg_write_reg_wait ring callback (v2)

This adds support for writing and reading back in a single
oneshot packet.  This is needed to send a tlb invalidation
and wait for ack in a single operation.

v2: squash the gfx ring stall fix

Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Emily Deng <Emily.Deng@amd.com>