drm/cirrus: Compute blit destination offset in single location

The calculation for the scanout-buffer blit offset is independent
from the color format. In the one case where the current code uses
fb->pitches[0] instead of cirrus->pitch, their values are identical.
Hence merge all into a single line.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-2-tzimmermann@suse.de