drm/amd/display: add mechanism to skip DCN init

drm/amd/display: add mechanism to skip DCN init

[Why]
If optimized init is done in FW. DCN init can be skipped in driver. This
need to be communicated between driver and fw and maintain backwards
compatibility.

[How]
Use DMUB scratch 0 bit 2 to indicate optimized init done in fw and
use DMUB scatch 4 bit 0 to indicate drive supports the optimized flow
so FW will perform it.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>