drm/i915/gt: Prefer IS_ENABLED() instead of defined() on config option

Use IS_ENABLED() instead of defined() for checking whether a kconfig
option is enabled.

Signed-off-by: Nitin Gote <nitin.r.gote@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250103062651.798249-2-nitin.r.gote@intel.com