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

drivers/gpu/drm/panel/panel-simple.c

@@ -1763,7 +1763,7 @@ static const struct drm_display_mode frida_frd350h54004_mode = {
 	.vsync_start = 240 + 2,
 	.vsync_end = 240 + 2 + 6,
 	.vtotal = 240 + 2 + 6 + 2,
-	.flags = DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC,
+	.flags = DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC,
 };
 
 static const struct panel_desc frida_frd350h54004 = {