drm/amdgpu/display: use msleep rather than udelay for long delays

Some architectures (e.g., ARM) throw an compilation error if the
udelay is too long.  In general udelays of longer than 2000us are
not recommended on any architecture.  Switch to msleep in these
cases.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>