drm/amdgpu: Extend full access wait time in guest

- Extend wait time and add retry, currently 6s * 2times
- Change timing algorithm

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>