drm/rcar-du: dsi: Handle both DRM_MODE_FLAG_N.SYNC and !DRM_MODE_FLAG_P.SYNC

Since commit 94fe479fae96 ("drm/rcar-du: dsi: Clean up handling of DRM mode flags")
the driver does not set TXVMVPRMSET0R_VSPOL_LOW and TXVMVPRMSET0R_HSPOL_LOW
for modes which set neither DRM_MODE_FLAG_[PN].SYNC. The previous behavior
was to assume that neither flag means DRM_MODE_FLAG_N.SYNC . Restore the
previous behavior for maximum compatibility.

The change of behavior is visible below, consider Vertical mode->flags
for simplicity sake, although the same applies to Horizontal ones:

Before 94fe479fae96 ("drm/rcar-du: dsi: Clean up handling of DRM mode flags") :

- DRM_MODE_FLAG_PVSYNC => vprmset0r |= 0
- DRM_MODE_FLAG_NVSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW
- Neither DRM_MODE_FLAG_[PN]VSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW

After 94fe479fae96 ("drm/rcar-du: dsi: Clean up handling of DRM mode flags") :

- DRM_MODE_FLAG_PVSYNC => vprmset0r |= 0
- DRM_MODE_FLAG_NVSYNC => vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW
- Neither DRM_MODE_FLAG_[PN]VSYNC => vprmset0r |= 0 <---------- This broke

The "Neither" case behavior is different, because DRM_MODE_FLAG_N[HV]SYNC is
really not equivalent !DRM_MODE_FLAG_P[HV]SYNC .

Fixes: 94fe479fae96 ("drm/rcar-du: dsi: Clean up handling of DRM mode flags")
Signed-off-by: Marek Vasut <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Tomi Valkeinen <[e-mail]>

drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c

···
 
 	/* Configuration for Video Parameters, input is always RGB888 */
 	vprmset0r = TXVMVPRMSET0R_BPP_24;
-	if (mode->flags & DRM_MODE_FLAG_NVSYNC)
+	if (!(mode->flags & DRM_MODE_FLAG_PVSYNC))
 		vprmset0r |= TXVMVPRMSET0R_VSPOL_LOW;
-	if (mode->flags & DRM_MODE_FLAG_NHSYNC)
+	if (!(mode->flags & DRM_MODE_FLAG_PHSYNC))
 		vprmset0r |= TXVMVPRMSET0R_HSPOL_LOW;
 
 	vprmset1r = TXVMVPRMSET1R_VACTIVE(mode->vdisplay)