drm/probe-helper: Add drm_connector_helper_get_modes_fixed()

Add drm_connector_helper_get_modes_fixed(), which duplicates a single
display mode for a connector. Convert drivers.

v2:
	* rename 'static' and 'hw' to 'fixed' everywhere
	* fix typo 'there' to 'their' (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220905141648.22013-2-tzimmermann@suse.de