drm/fbdev-dma: Implement damage handling and deferred I/O

Add support for damage handling and deferred I/O to fbdev-dma. This
enables fbdev-dma to support all DMA-memory-based DRM drivers, even
such with a dirty callback in their framebuffers.

The patch adds the code for deferred I/O and also sets a dedicated
helper for struct fb_ops.fb_mmap that support coherent mappings.

v3:
- init fb_ops with FB_GEN_DEFAULT_DEFERRED_DMAMEM_OPS() (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-22-tzimmermann@suse.de