drm/amd/display: Reset inbox rptr/wptr when resetting DMCUB

[Why]
These logically make sense more to be set after the DMCUB has been
reset rather than when we setup the inbox.

[How]
Move them into the reset callback.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>