drm/amd/display: set minimum of VBlank_nom

[Why]
If VBlank_nom is too small, it will cause
VStartUP_Start smaller than VBackPorch + VSync width which is an
invalid case for VStartUP_Start and where to send AS-SDP.

[How]
Setup a minimum value to VBlank_nom

Reviewed-by: Reza Amini <reza.amini@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: ChunTao Tso <chuntao.tso@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>