drm/amd/display: Prevent using DMUB rptr that is out-of-bounds

[Why]
Running into bugchecks during stress test where rptr is 0xFFFFFFFF.
Typically this is caused by a hard hang, and can come from HW outside
of DCN.

[How]
To prevent bugchecks when writing the DMUB rptr, fist check that the
rptr is valid.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>