modesetting: Use GBM_FORMAT_XRGB8888 for 24-bit depth front bo's

Same as what we use in glamor/glamor_egl.c

Signed-off-by: stefan11111 <stefan11111@shitposting.expert>

hw/xfree86/drivers/video/modesetting/drmmode_display.c

@@ -1174,6 +1174,8 @@ drmmode_gbm_format_for_depth(int depth)
         return GBM_FORMAT_ARGB1555;
     case 16:
         return GBM_FORMAT_RGB565;
+    case 24:
+        return GBM_FORMAT_XRGB8888;
     case 30:
         /* XXX Is this format right? https://github.com/X11Libre/xserver/pull/1396/files#r2523698616 XXX */
         return GBM_FORMAT_ARGB2101010;