drm/amd/display: Fix DCN42 memory clock table using MemClk instead of UClk

drm/amd/display: Fix DCN42 memory clock table using MemClk instead of UClk

[Why]
DCN42 was using UClk values instead of MemClk from MemPstateTable, causing
DML to see half the actual DRAM bandwidth on DDR5 systems and reject high
refresh rate modes.

[How]
Change dcn42_init_clocks() to use MemPstateTable[i].MemClk instead of
MemPstateTable[i].UClk for memclk_mhz initialization.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alexander Chechik <alexander.chechik@amd.com>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>