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

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>
Cc: stable@vger.kernel.org