drm/i915: switch intel_wakeref_t underlying type to struct ref_tracker *

drm/i915: switch intel_wakeref_t underlying type to struct ref_tracker *

For intel_wakeref_t, opaque is reasonable, but disguising the underlying
struct ref_tracker * as an unsigned long is not so great. Update the
typedef to remove one level of disguise.

Although the kernel coding style strongly discourages pointer typedefs,
it's a better alternative, and an incremental improvement on the status
quo. It provides much better type safety than an unsigned long could,
and prevents passing magic -1 instead of INTEL_WAKEREF_DEF. Moreover, it
provides a gradual path for replacing intel_wakeref_t with struct
ref_tracker * if desired.

As an extra safety measure, check for error pointers in
intel_ref_tracker_free() before passing them on to ref_tracker_free(),
to catch any mistakes with mock gt special wakeref value.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cca2b0631f816ad90461aa1bf4fe3f80c0e13464.1726680898.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>