drm/amd/display: refactor vpg.h

why and how:
as part of cleanup, need to refactor vpg. It was improperly referenced
as a dcn specfic part of link. the dcn agnostic code needed was ripped out
and put into vpg.h, now in dc/inc/hw.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Parandhaman K <parandhaman.k@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>