drm/mipi-dbi: Initialize default driver functions with macro

Introduce DRM_MIPI_DBI_SIMPLE_DISPLAY_PIPE_FUNCS to initialize MIPI-DBI
helpers to default values and convert drivers. The prepare_fb function
set by some drivers is called implicitly by simple-kms helpers, so leave
it out.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org> # drm/tiny/mi0283qt
Link: https://patchwork.freedesktop.org/patch/msgid/20221202125644.7917-5-tzimmermann@suse.de