drm/amd/display: Reset DMUB mailbox SW state after HW reset

[Why]
Otherwise we can be out of sync with what's in the hardware, leading
to us rerunning every command that's presently in the ringbuffer.

[How]
Reset software state for the mailboxes in hw_reset callback.
This is already done as part of the mailbox init in hw_init, but we
do need to remember to reset the last cached wptr value as well here.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Acked-by: Alex Hung <alex.hung@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>