drm/amd/display: combine public interfaces into single header

[Why]
We want to better encapsulate all driver-fw dependencies into a single
file.

[How]
Combine all the headers under inc folder into a single header

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>