drm/omapdrm: Fix console by implementing fb_dirty

The framebuffer console stopped updating with commit f231af49
("drm/fb-helper: Disconnect damage worker from update logic").

Let's fix the issue by implementing fb_dirty similar to what was done
with commit 039a72ce ("drm/i915/fbdev: Implement fb_dirty for intel
custom fb helper").

Fixes: f231af49 ("drm/fb-helper: Disconnect damage worker from update logic")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228063540.4444-2-tony@atomide.com