bhyve amd: Small cleanups in amdvi_dump_cmds

Bump offset with MOD_INC instead in amdvi_dump_cmds.

Reviewed by: jhb
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D28862

(cherry picked from commit be97fc8dced052f824387a1ae6a0063f9dcfdc89)