drm/i915/guc: Replace zero-length arrays with flexible-array members

Zero-length arrays are deprecated[1] and we are moving towards
adopting C99 flexible-array members, instead. So, replace zero-length
arrays in a couple of structures (three, actually) with flex-array
members.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [2].

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays [1]
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [2]
Link: https://github.com/KSPP/linux/issues/78


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Y72WBTUmh9r1lvKN@work