drm/amd/display: make seamless boot work generically

[Why]
Seamless boot code not working on all ASICs because of
some underflow issues caused by some uninitialized HW
state.

[How]
Keep some logical and power gating init code in hw_init.
Move some per pipe init code to enable accelerated mode

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