drm/amdgpu/dm/mst: Rename get_payload_table()

This function isn't too confusing if you see the comment around the
call-site for it, but if you don't then it's not at all obvious this is
meant to copy DRM's payload table over to DC's internal state structs.
Seeing this function before finding that comment definitely threw me into a
loop a few times.

So, let's rename this to make it's purpose more obvious regardless of where
in the code you are.

Signed-off-by: Lyude Paul <lyude@redhat.com>