panfrost: fix strict-aliasing violations when packing fb ptrs

panfrost: fix strict-aliasing violations when packing fb ptrs

Compilers are free to make the assumption that pointers don't violate
strict aliasing. If that assumption is incorrect, as it is with the
framebuffer pointer packing code here, the job can fail.

This depends heavily on the compiler and optimization levels, so it's
hard to reproduce, but it did happen for at least two users running with
-O2 on gcc.

Fixes: 67cbbf941751 ("panfrost: Use framebuffer pointer XML")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8627
Signed-off-by: Italo Nicola <italonicola@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21991>