drm/panel-simple: Fix inverted V/H SYNC for Frida FRD350H54004 panel

The FRD350H54004 panel was marked as having active-high VSYNC and HSYNC
signals, which sorts-of worked, but resulted in the picture fading out
under certain circumstances.

Fix this issue by marking VSYNC and HSYNC signals active-low.

v2: Rebase on drm-misc-next

Fixes: 7b6bd843 ("drm/panel: simple: Add support for the Frida FRD350H54004 panel")
Cc: stable@vger.kernel.org # v5.5
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125647.10964-1-paul@crapouillou.net