drm/amdgpu: fix and cleanup gmc_v9_0_flush_gpu_tlb

The KIQ code path was ignoring the second flush. Also avoid long lines and
re-calculating the register offsets over and over again.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>