drm/amd/display: add separate of private hwss functions

[Why]
Some function pointers in the hwss function pointer table are
meant to be hw sequencer entry points to be called from dc.

However some of those function pointers are not meant to
be entry points, but instead used as a code reuse/inheritance
tool called directly by other hwss functions, not by dc.

Therefore, we want a more clear separation of which functions
we determine to be interface functions vs the functions we
use within hwss.

[How]
DC interface functions will be stored in:
    struct hw_sequencer_funcs
Functions used within HWSS will be stored in:
    struct hwseq_private_funcs

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