drm/i915/gvt: Rework fix out-of-bounds buffer write into opregion->signature

[Nicolas Chauvet]

With gcc-15, there is a new warning treated as error
    As shown with gcc-15:
    drivers/gpu/drm/i915/gvt/opregion.c: In function intel_vgpu_init_opregion:
    drivers/gpu/drm/i915/gvt/opregion.c:35:28: error: initializer-string for array of char is too long [-Werror=unterminated-string-initialization]
       35 | #define OPREGION_SIGNATURE "IntelGraphicsMem"
          |                            ^~~~~~~~~~~~~~~~~~
    drivers/gpu/drm/i915/gvt/opregion.c:225:45: note: in expansion of macro OPREGION_SIGNATURE
      225 |         const char opregion_signature[16] = OPREGION_SIGNATURE;
          |                                             ^~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors

Rework the previous fix by reverting it, then two options:
- Enlarge the signature field
- Reduce the string size 

I would use the latter, but I don't know the runtime implication of it
(hence the RFC).

This issue was discovered by building longterm kernel-6.12 in newer
fedora, See also:
https://copr.fedorainfracloud.org/coprs/kwizart/kernel-longterm-6.12/

Signed-off-by: Nicolas Chauvet <kwizart at gmail.com>