drm/scheduler: stop setting rq to NULL

We removed the redundancy of having an extra scheduler field, so we
can't set the rq to NULL any more or otherwise won't know which
scheduler to use for the cleanup.

Just remove the entity from the scheduling list instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107367


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>