drm/i915: Reduce ELD hex dumps a bit

drm/i915: Reduce ELD hex dumps a bit

Do the ELD hexdumps only up to the last differing byte.
The rest is typically all zeroes anyway so not much point
in dumping it.

Couldn't find anything for memcmp_diff_len() so
rolled my own.

v2: Use semantics and function name suggested by Jani

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215150129.13288-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>