drm/amd/display: Use mdelay to avoid context switch

[why]
The rapid msleep operation causes the white line garbage when
DAL check flip pending status in SetVidPnSourceVisibility.
To execute this msleep will induce context switch, and longer
delay could cause worse garbage situation.

[how]
To replace msleep with mdelay.

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>