drm/cirrus: Inline cirrus_fb_blit_rect()

Inline cirrus_fb_blit_rect into its only caller. While at it, update
the code to use IOSYS_MAP_INIT_OFFSET(), which is the ideomatic way
of initializing struct iosys_map with an offset.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-11-tzimmermann@suse.de