drm/cirrus: Split cirrus_mode_set() into smaller functions

Split cirrus_mode_set() into smaller functions that set the display
mode, color format and scnaline pitch individually. Better reflects
the design of the DRM modesetting pipeline.

Done in preparation of converting cirrus to regular atomic helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-6-tzimmermann@suse.de