drm/arm/malidp: Preserve LAYER_FORMAT contents when setting format

On some Mali-DP processors, the LAYER_FORMAT register contains fields
other than the format. These bits were unconditionally cleared when
setting the pixel format, whereas they should be preserved at their
reset values.
Reported-by: Brian Starkey <brian.starkey@arm.com>
Reported-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Ayan Kumar halder <ayan.halder@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>

drivers/gpu/drm/arm/malidp_planes.c

@@ -23,6 +23,7 @@
 
 /* Layer specific register offsets */
 #define MALIDP_LAYER_FORMAT		0x000
+#define   LAYER_FORMAT_MASK		0x3f
 #define MALIDP_LAYER_CONTROL		0x004
 #define   LAYER_ENABLE			(1 << 0)
 #define   LAYER_FLOWCFG_MASK		7
@@ -337,7 +338,9 @@ static void malidp_de_plane_update(struct drm_plane *plane,
 	dest_w = plane->state->crtc_w;
 	dest_h = plane->state->crtc_h;
 
-	malidp_hw_write(mp->hwdev, ms->format, mp->layer->base);
+	val = malidp_hw_read(mp->hwdev, mp->layer->base);
+	val = (val & ~LAYER_FORMAT_MASK) | ms->format;
+	malidp_hw_write(mp->hwdev, val, mp->layer->base);
 
 	for (i = 0; i < ms->n_planes; i++) {
 		/* calculate the offset for the layer's plane registers */