drm/i915/gt: Fix SPDX license format

Header files need to declare the SPDX under /* ... */ style
comments at the beginning of the file.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250314021225.11813-3-andi.shyti@linux.intel.com