drm/panel: simple: Fix g121x1_l03 hsync/vsync polarity

Set hsync/vsync to active low for g121x1_l03 panel to match the
recommended setting in the datasheet.

Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>