compositor-drm: Remove unnecessary picked_scanout variable

e2e80136 fixed the same issue as df573031 in a different way.
The latter commit (applied earlier in the upstream tree) adds a variable
to assign_planes to keep track of when we successfully assign a view to
the scanout plane, and doesn't call prepare_scanout_view if we have.

The former commit adds this checking inside prepare_scanout_view: if the
pending output state already has a framebuffer assigned to the scanout
plane, we drop out of prepare_scanout_view early. The picked_scanout
variable inside assign_planes can thus be removed.

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