amdgpu: securedisplay: simplify i2c hexdump output

A previous fix I did left a rather complicated loop in
amdgpu_securedisplay_debugfs_write() for what could be expressed in a
simple sprintf, as Rasmus pointed out.

This drops the leading 0x for each byte, but is otherwise
much nicer.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>