drm/amd/display: Fix eDP will flash when boot to OS

[WHY]
With eDP DSC enabled and set 4K 60Hz, there would be screen
corruption when booting to OS or enabling the driver.

[HOW]
Avoid powering down VDD when we cannot apply eDP fast boot.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>