drm/amdgpu: check RAS supported first in ras_reset_error_count

Not all platforms support RAS.

Fixes: 73582be1 ("drm/amdgpu: bypass RAS error reset in some conditions")
Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>