drm/panthor: Clean up FW version information display

drm/panthor: Clean up FW version information display

Assigning a string to an array which is too small to include the NUL
byte at the end causes a warning on some compilers. But this function
also has some other oddities like the 'header' array which is only ever
used within sizeof().

Tidy up the function by removing the 'header' array, allow the NUL byte
to be present in git_sha_header, and calculate the length directly from
git_sha_header.

Reported-by: Will Deacon <will@kernel.org>
Closes: https://lore.kernel.org/all/20250213154237.GA11897@willie-the-truck/
Fixes: 9d443deb0441 ("drm/panthor: Display FW version information")
Signed-off-by: Steven Price <steven.price@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250213161248.1642392-1-steven.price@arm.com