drm/amd/display: Prevent VStartup Overflow

[WHY & HOW]
Fixed Overflow issue by clamping VStartup to max value of register.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Ryan Seto <ryanseto@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>