drm/amd/display: Use udelay when waiting between aux retries

[Why]
"IRQ_HPD Pulse Length Test" DP compliance test
fails. Test complains that certain DPCD registers
are not read within 100 ms.

[How]
msleep is inaccurate for small values. Used udelay
instead for accuracy.

Signed-off-by: John Barberiz <John.Barberiz@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>