drm/amd/display: make dc_set_power_state() return type `void` again

As dc_set_power_state() no longer allocates memory, it's not necessary
to have return types and check return code as it can't fail anymore.

Change it back to `void`.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>