drm/amd/display: Add power down display on boot flag

[Why]

Due to the generic introduction of seamless boot, the display is no
longer blanked upon boot. However, this causes corruption on some
systems that does not lock the memory in the non-secure boot case,
resulting in brief corruption on boot due to garbage being written into
the frame buffer.

[How]
Add a flag, read during DC init, to determine whether display should be
blanked on boot. Default to true.

Signed-off-by: Thomas Lim <Thomas.Lim@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>