drm/amd/display: really fix time out in init sequence

REG_UPDATE_2 return the reg value it write out through MMIO
we need to do a REG_READ to confirm the value is written out

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>