drm/amd/display: use udelay rather than fsleep

This function can be called from an atomic context so we can't use
fsleep().

Fixes: 01f60348 ("drm/amd/display: Fix 'failed to blank crtc!'")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4549


Cc: Wen Chen <Wen.Chen3@amd.com>
Cc: Fangzhi Zuo <jerry.zuo@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>