drm/amd/display: Disabled seamless boots on DP and renamed power_down_display_on_boot

[WHY]
- We only ever want seamless boots on eDPs
- The naming and logic did not match the context

[HOW]
- Removed unnecessary if statements
- Renamed power_down_display_on_boot to seamless_boot_edp_requested and
swapped the logic

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jarif Aftab <jaraftab@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>