drm/amd/display: Increase timeout threshold for DMCUB reset

[Why]
If we're backdoor loading the DMCUB performs more work than just
the PHY reset so we can end up resetting before the cleanup has fully
finished.

[How]
Increase timeout, add udelay between spins to guarantee a minimum.

Reviewed-by: Roy Chan <roy.chan@amd.com>
Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>