drm/panthor: Fix clang -Wunused-but-set-variable in tick_ctx_apply()

drm/panthor: Fix clang -Wunused-but-set-variable in tick_ctx_apply()

Clang warns (or errors with CONFIG_WERROR):

  drivers/gpu/drm/panthor/panthor_sched.c:2048:6: error: variable 'csg_mod_mask' set but not used [-Werror,-Wunused-but-set-variable]
   2048 |         u32 csg_mod_mask = 0, free_csg_slots = 0;
        |             ^
  1 error generated.

The variable is an artifact left over from refactoring that occurred
during the development of the initial series for this driver. Remove it
to resolve the warning.

Fixes: de8548813824 ("drm/panthor: Add the scheduler logical block")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328-panthor-drop-csg_mod_mask-v1-1-5a80be3df581@kernel.org