drm/scheduler: modify API to avoid redundancy

entity has a scheduler field and we don't need the sched argument
in any of the functions where entity is provided.

Signed-off-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>