drm/amd/display: Prevent VStartup Overflow

[Why]
For some VR headsets with large blanks, it's possible
to overflow the OTG_VSTARTUP_PARAM:VSTARTUP_START
register. This can lead to incorrect DML calculations
and underflow downstream.

[How]
Min the calcualted max_vstartup_lines with the max
value of the register.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Ryan Seto <ryanseto@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>