drm/scheduler: implement a function to modify sched list

Implement drm_sched_entity_modify_sched() which modifies existing
sched_list with a different one. This is going to be helpful when
userspace changes priority of a ctx/entity then the driver can switch
to the corresponding HW scheduler list for that priority.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>