drm/radeon: make sure vertical front porch is at least 1

[ Upstream commit 3104b812 ]

hw doesn't like a 0 value.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>