drm/amd/display: export some optc function for reuse

[Why&How]
Make a few functions non static so that they can be reused for other
asic. This is in preparation for separating out OTG programming sequence
for DCN301

Fixes: 1598fc57

 ("drm/amd/display: Program OTG vtotal min/max selectors unconditionally for DCN1+")
Reviewed-by: Swapnil Patel <swapnil.patel@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>