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

commit 3104b8128d4d646a574ed9d5b17c7d10752cd70b upstream.

hw doesn't like a 0 value.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/gpu/drm/radeon/atombios_encoders.c
Simple merge