compositor-drm: Fully account for buffer transformation

In our new and improved helper to determine the src/dest values for a
buffer on a given plane, make sure we account for all buffer
transformations, including viewport clipping.

Rather than badly open-coding it ourselves, just use the helper which
does exactly this.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reported-by: Tiago Gomes <tiago.gomes@codethink.co.uk>
Tested-by: Emre Ucan <eucan@de.adit-jv.com>