drm/mediatek: Set sensible cursor width/height values to fix crash

Hardware-speaking, there is no feature-reduced cursor specific
plane, so this driver reserves the last all Overlay plane as a
Cursor plane, but sets the maximum cursor width/height to the
maximum value that the full overlay plane can use.

While this could be ok, it raises issues with common userspace
using libdrm (especially Mutter, but other compositors too) which
will crash upon performing allocations and/or using said cursor
plane.

Reduce the maximum width/height for the cursor to 512x512 pixels,
value taken from IGT's maximum cursor size test, which succeeds.

Signed-off-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Fei Shao <[e-mail]>
Tested-by: Fei Shao <[e-mail]>
Reviewed-by: Daniel Stone <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[e-mail]/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

···
 	}
 
 	/* IGT will check if the cursor size is configured */
-	drm->mode_config.cursor_width = drm->mode_config.max_width;
-	drm->mode_config.cursor_height = drm->mode_config.max_height;
+	drm->mode_config.cursor_width = 512;
+	drm->mode_config.cursor_height = 512;
 
 	/* Use OVL device for all DMA memory allocations */
 	crtc = drm_crtc_from_index(drm, 0);