drm/fbdev: fix drm_fb_helper_deferred_io() build failure

The drm_fb_helper_deferred_io() uses struct fb_deferred_io_pageref,
which isn't available without CONFIG_FB_DEFERRED_IO. Put the function
under corresponding #ifdef to fix build failure if deferred I/O isn't
enabled.

Fixes: 8058944f ("drm/fbdev: Select fbdev I/O helpers from modules that require them")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021-fix-drm-deferred-v2-1-db1de4c6b042@linaro.org