drm/amd/display: Prevent vtotal from being set to 0

[Why]
In dcn314 DML the destination pipe vtotal was being set
to the crtc adjustment vtotal_min value even in cases
where that value is 0.

[How]
Only set vtotal to the crtc adjustment vtotal_min value
in cases where the value is non-zero.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>