drm/amd/display: Fixed Intermittent blue screen on OLED panel

[why]
Changing to smaller modes on OLED panel caused a blue screen crash
as driver reported dram change during vactive when it shouldn't

[how]
Added an extra condition to prevent incorrect dram change timing

Signed-off-by: Naveed Ashfaq <Naveed.Ashfaq@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>