drm/i915/bios: Fix the timing parameters

Fix htotal and vtotal parameters derived from DTD block of VBT. The
values miss the back porch.

Fixes: 33ef6d4f ("drm/i915/vbt: Handle generic DTD block")
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124125829.16973-1-vandita.kulkarni@intel.com