drm/i915: Add some essential functionality for joiners

In most of the cases we now try to avoid mentioning things like
"bigjoiner" or "ultrajoiner" trying to unify the API and refer
mostly to all this functionality as "joiner".
In majority cases that should be way to go.
However in some cases we still need to distinguish between
bigjoiner primaries and secondaries(such as DSC register programming).

Create correspondent helper functions and start using them,
in order be prepared for adding ultrajoiner functionality.

v2: Fixed checkpatch warnings (Ankit)
v3: Introduce ultrajoiner helpers in next patch.
v4: Streamline the helpers and add few more. (Ville)
v5: Add comment to clarify that helpers apply to both bigjoiner and
uncompressed joiner configurations. (Ville)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240918063016.2667721-2-ankit.k.nautiyal@intel.com