drm/amd/display: Fix dmub_psr_destroy()

This is freeing the wrong variable so it will crash.  It should be
freeing "*dmub" instead of "dmub".

Fixes: 4c1a1335 ("drm/amd/display: Driverside changes to support PSR in DMCUB")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>