drm/amd/display: Use Nominal vBlank If Provided Instead Of Capping It

[Why/How]
vBlank used to determine the max vStartup is based on the smallest between
the vblank provided by the timing and vblank in ip_caps.
Extra vblank time is not considered if the vblank provided by the timing ends
up being higher than what's defined by the ip_caps

Use 1 less than the vblank size in case the timing is interlaced
so vstartup will always be less than vblank_nom.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Austin Zheng <Austin.Zheng@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>