drm/i915: Use container_of_const() for states

drm/i915: Use container_of_const() for states

commit 64f6a5d1922b ("container_of: add container_of_const()
that preserves const-ness of the pointer") is nice. Let's use
it so that we don't accidentally cast away the const from our
state pointers.

The only thing I don't particularly like about container_of_const()
is that it still accepts void* in addition to the proper pointer
types, but that's how most other things in C work anyway so I
guess we can live with it.

And while at it rename the macro arguments to be a bit more
descriptive than just 'x'.

TODO: maybe convert *all* container_of() uses to container_of_const()?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240307151810.24208-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>