drm/v3d: Annotate struct v3d_perfmon with __counted_by

drm/v3d: Annotate struct v3d_perfmon with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct v3d_perfmon.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Emma Anholt <emma@anholt.net>
Cc: Melissa Wen <mwen@igalia.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230922173216.3823169-9-keescook@chromium.org