[Why]
We wait for mpc idle while in a locked state, leading to potential
deadlock.

[What]
Move the wait_for_idle call to outside of HW lock. This and a
call to wait_drr_doublebuffer_pending_clear are moved added to a new
static helper function called wait_for_outstanding_hw_updates, to make
the interface clearer.

Cc: stable@vger.kernel.org
Fixes: 8f0d304d ("drm/amd/display: Do not commit pipe when updating DRR")
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Gabe Teeger <gabe.teeger@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>