drm/amd/display: Change amdgpu_dm_irq_suspend() to void

amdgpu_dm_irq_suspend() doesn't have any error flows and always
returns zero.

Change the function to void.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>