drm/radeon/combios: add some validation of lvds values

commit 0a90a0cf upstream.

Fixes a broken hsync start value uncovered by:
abc0b144
(drm: Perform basic sanity checks on probed modes)

The driver handled the bad hsync start elsewhere, but
the above commit prevented it from getting added.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=91401



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>