drm/amdgpu: fix initialization of doorbell array for detect and hang

Initialized doorbells should be set to invalid rather than 0 to prevent
driver from over counting hung doorbells since it checks against the
invalid value to begin with.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>