drm/panic: use `div_ceil` to clean Clippy warning

Starting with the upcoming Rust 1.86.0 (to be released 2025-04-03),
Clippy warns:

    error: manually reimplementing `div_ceil`
       --> drivers/gpu/drm/drm_panic_qr.rs:548:26
        |
    548 |         let pad_offset = (offset + 7) / 8;
        |                          ^^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `offset.div_ceil(8)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_div_ceil

And similarly for `stride`. Thus apply the suggestion to both.

The behavior (and thus codegen) is not exactly equivalent [1][2], since
`div_ceil()` returns the right value for the values that currently
would overflow.

Link: https://github.com/rust-lang/rust-clippy/issues/14333 [1]
Link: https://godbolt.org/z/dPq6nGnv3

 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Fixes: cb5164ac ("drm/panic: Add a QR code panic screen")
Cc: stable@vger.kernel.org # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250301231602.917580-1-ojeda@kernel.org