issue:
the UMC6 h/w bug is that when MCLK is doing the switch
in the middle of a page access being preempted by high
priority client (e.g. DISPLAY) then UMC and the mclk switch
would stuck there due to deadlock

how:
fixed by disabling auto PreChg for UMC to avoid high
priority client preempting other client's access on
the same page, thus the deadlock could be avoided

v2:
put the patch in callback of UMC6
v3:
rename the callback to "init_registers"
Signed-off-by: NMonk Liu <Monk.Liu@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NHawking Zhang <hawking.zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>