drm/amd/display: cleaning up hubp for dcn

removing duplicate functions, renaming struct mi_regs, etc.

Signed-off-by: Yue Hin Lau <Yuehin.Lau@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>