drm/msm: Restore second parameter name in purge() and evict()

drm/msm: Restore second parameter name in purge() and evict()

After commit 3392291fc509 ("drm/msm: Fix shrinker deadlock"), all
supported versions of clang warn (or error with CONFIG_WERROR=y):

  drivers/gpu/drm/msm/msm_gem_shrinker.c:105:58: error: omitting the parameter name in a function definition is a C23 extension [-Werror,-Wc23-extensions]
    105 | purge(struct drm_gem_object *obj, struct ww_acquire_ctx *)
        |                                                          ^
  drivers/gpu/drm/msm/msm_gem_shrinker.c:117:58: error: omitting the parameter name in a function definition is a C23 extension [-Werror,-Wc23-extensions]
    117 | evict(struct drm_gem_object *obj, struct ww_acquire_ctx *)
        |                                                          ^
  2 errors generated.

With older but supported versions of GCC, this is an unconditional hard error:

  drivers/gpu/drm/msm/msm_gem_shrinker.c: In function 'purge':
  drivers/gpu/drm/msm/msm_gem_shrinker.c:105:35: error: parameter name omitted
   purge(struct drm_gem_object *obj, struct ww_acquire_ctx *)
                                     ^~~~~~~~~~~~~~~~~~~~~~~
  drivers/gpu/drm/msm/msm_gem_shrinker.c: In function 'evict':
  drivers/gpu/drm/msm/msm_gem_shrinker.c:117:35: error: parameter name omitted
   evict(struct drm_gem_object *obj, struct ww_acquire_ctx *)
                                     ^~~~~~~~~~~~~~~~~~~~~~~

Restore the parameter name to clear up the warnings, renaming it
"unused" to make it clear it is only needed to satisfy the prototype of
drm_gem_lru_scan().

Cc: stable@vger.kernel.org
Fixes: 3392291fc509 ("drm/msm: Fix shrinker deadlock")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>