drm/amdgpu: Modify indirect register access for gfx9 sriov

Expand RLCG interface for new GC read & write commands.
New interface will only be used if the PF enables the flag in pf2vf msg.

v2: Added a description for the scratch registers

Signed-off-by: Victor Skvortsov <victor.skvortsov@amd.com>
Reviewed-by: David Nieto <david.nieto@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>