drm/imx: ipuv3-plane: fix PRG modifiers after drm managed resource conversion

The conversion to drm managed resources introduced two bugs: the plane is now
always initialized with the linear-only list, while the list with the Vivante
GPU modifiers should have been used when the PRG/PRE engines are present. This
masked another issue, as ipu_plane_format_mod_supported() is now called before
the private plane data is set up, so if a non-linear modifier is supplied in
the plane modifier list, we run into a NULL pointer dereference checking for
the PRG presence. To fix this just remove the check from this function, as we
know that it will only be called with a non-linear modifier, if the plane init
code has already determined that the PRG/PRE is present.

Fixes: 699e7e54 ("drm/imx: ipuv3-plane: use drm managed resources")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20210510145927.988661-1-l.stach@pengutronix.deSigned-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/drm/imx/ipuv3-plane.c

@@ -349,10 +349,11 @@ static bool ipu_plane_format_mod_supported(struct drm_plane *plane,
 	if (modifier == DRM_FORMAT_MOD_LINEAR)
 		return true;
 
-	/* without a PRG there are no supported modifiers */
-	if (!ipu_prg_present(ipu))
-		return false;
-
+	/*
+	 * Without a PRG the possible modifiers list only includes the linear
+	 * modifier, so we always take the early return from this function and
+	 * only end up here if the PRG is present.
+	 */
 	return ipu_prg_format_supported(ipu, format, modifier);
 }
 
@@ -878,6 +879,10 @@ struct ipu_plane *ipu_plane_init(struct drm_device *dev, struct ipu_soc *ipu,
 		formats = ipu_plane_rgb_formats;
 		format_count = ARRAY_SIZE(ipu_plane_rgb_formats);
 	}
+
+	if (ipu_prg_present(ipu))
+		modifiers = pre_format_modifiers;
+
 	ipu_plane = drmm_universal_plane_alloc(dev, struct ipu_plane, base,
 					       possible_crtcs, &ipu_plane_funcs,
 					       formats, format_count, modifiers,
@@ -892,9 +897,6 @@ struct ipu_plane *ipu_plane_init(struct drm_device *dev, struct ipu_soc *ipu,
 	ipu_plane->dma = dma;
 	ipu_plane->dp_flow = dp;
 
-	if (ipu_prg_present(ipu))
-		modifiers = pre_format_modifiers;
-
 	drm_plane_helper_add(&ipu_plane->base, &ipu_plane_helper_funcs);
 
 	if (dp == IPU_DP_FLOW_SYNC_BG || dp == IPU_DP_FLOW_SYNC_FG)