drm/fbdev-dma: Select FB_DEFERRED_IO

Commit 808a40b ("drm/fbdev-dma: Implement damage handling and
deferred I/O") added deferred I/O for fbdev-dma. Also select the
Kconfig symbol FB_DEFERRED_IO (via FB_DMAMEM_HELPERS_DEFERRED). Fixes
build errors about missing fbdefio, such as

drivers/gpu/drm/drm_fbdev_dma.c:218:26: error: 'struct drm_fb_helper' has no member named 'fbdefio'
  218 |                 fb_helper->fbdefio.delay = HZ / 20;
      |                          ^~
drivers/gpu/drm/drm_fbdev_dma.c:219:26: error: 'struct drm_fb_helper' has no member named 'fbdefio'
  219 |                 fb_helper->fbdefio.deferred_io = drm_fb_helper_deferred_io;
      |                          ^~
drivers/gpu/drm/drm_fbdev_dma.c:221:21: error: 'struct fb_info' has no member named 'fbdefio'
  221 |                 info->fbdefio = &fb_helper->fbdefio;
      |                     ^~
drivers/gpu/drm/drm_fbdev_dma.c:221:43: error: 'struct drm_fb_helper' has no member named 'fbdefio'
  221 |                 info->fbdefio = &fb_helper->fbdefio;
      |                                           ^~

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410050241.Mox9QRjP-lkp@intel.com/
Fixes: 808a40b ("drm/fbdev-dma: Implement damage handling and deferred I/O")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: <stable@vger.kernel.org> # v6.11+
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241014085740.582287-4-tzimmermann@suse.de

drivers/gpu/drm/Kconfig

@@ -339,7 +339,7 @@ config DRM_TTM_HELPER
 config DRM_GEM_DMA_HELPER
 	tristate
 	depends on DRM
-	select FB_DMAMEM_HELPERS if DRM_FBDEV_EMULATION
+	select FB_DMAMEM_HELPERS_DEFERRED if DRM_FBDEV_EMULATION
 	help
 	  Choose this if you need the GEM DMA helper functions