drm/amdgpu/discovery: Replace fake flex-arrays with flexible-array members

drm/amdgpu/discovery: Replace fake flex-arrays with flexible-array members

Zero-length and one-element arrays are deprecated, and we are moving
towards adopting C99 flexible-array members, instead.

Use the DECLARE_FLEX_ARRAY() helper macro to transform zero-length
arrays in a union into flexible-array members. And replace a one-element
array with a C99 flexible-array member.

Address the following warnings found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1009:89: warning: array subscript kk is outside array bounds of ‘uint32_t[0]’ {aka ‘unsigned int[]’} [-Warray-bounds=]
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1007:94: warning: array subscript kk is outside array bounds of ‘uint64_t[0]’ {aka ‘long long unsigned int[]’} [-Warray-bounds=]
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1310:94: warning: array subscript k is outside array bounds of ‘uint64_t[0]’ {aka ‘long long unsigned int[]’} [-Warray-bounds=]
drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1309:57: warning: array subscript k is outside array bounds of ‘uint32_t[0]’ {aka ‘unsigned int[]’} [-Warray-bounds=]

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 [1].

This results in no differences in binary output.

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/300
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>