drm/amd: cleanup get_mfd_cell_dev()

It's simpler to just use snprintf() to print this to one buffer instead
of using strcpy() and strcat().  Also using snprintf() is slightly safer
than using sprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>