drm/amdgpu/display: switch udelay to msleep

drm/amdgpu/display: switch udelay to msleep

We may need to sleep for up to 80ms
(8ms per each of up to 10 loop iterations):

/* First DPCD read after VDD ON can fail if the particular board
 * does not have HPD pin wired correctly. So if DPCD read fails,
 * which it should never happen, retry a few times. Target worst
 * case scenario of 80 ms.
 */

Switch udelay to msleep to avoid limits on arm.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>