drm/format-helper: Add drm_fb_xrgb8888_to_xrgb2101010_toio()

Add XRGB8888 emulation support for devices that can only do XRGB2101010.

This is chiefly useful for simpledrm on Apple devices where the
bootloader-provided framebuffer is 10-bit.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211212062407.138309-3-marcan@marcan.st