drm/amdgpu: Modify indirect register access for gmc_v9_0 sriov

Modify GC register access from MMIO to RLCG if the
indirect flag is set

v2: Replaced ternary operator with if-else for better
readability

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>