drm/format-helper: Add blitter functions

The blitter functions copy a framebuffer to I/O memory using one of
the existing conversion functions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-3-tzimmermann@suse.de