drm/radeon: make -fstrict-flex-arrays=3 happy

The driver parses a union where the layout up through the first
array is the same, however, the array has different sizes
depending on the elements in the union.  Be explicit to
fix the UBSAN checker.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3323
Fixes: df8fc4e9

 ("kbuild: Enable -fstrict-flex-arrays=3")
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Kees Cook <keescook@chromium.org>