drm/amd/pm: fix BUG: scheduling while atomic

atomic scheduling will be triggered in interrupt handler for
AC/DC mode switch as following backtrace.
Call Trace:
 <IRQ>
 dump_stack_lvl
 __schedule_bug
 __schedule
 schedule
 schedule_preempt_disabled
 __mutex_lock
 smu_cmn_send_smc_msg_with_param
 smu_v13_0_irq_process
 amdgpu_irq_dispatch
 amdgpu_ih_process
 amdgpu_irq_handler
 __handle_irq_event_percpu
 handle_irq_event
 handle_edge_irq
 __common_interrupt
 common_interrupt
 </IRQ>
 <TASK>
 asm_common_interrupt

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Kun Liu <Kun.Liu2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>