drm/amd/display: Set the power_down_on_boot function pointer to null

drm/amd/display: Set the power_down_on_boot function pointer to null

[WHY]
Blackscreen hang @ PC EF000025 when trying to wake up from S0i3. DCN
gets powered off due to dc_power_down_on_boot() being called after
timeout.

[HOW]
Setting the power_down_on_boot function pointer to null since we don't
expect the function to be called for APU.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>