drm/amd/powerplay: fix off-by-one upper bounds limit checks

There are two occurrances of off-by-one upper bound checking of indexes
causing potential out-of-bounds array reads. Fix these.

Addresses-Coverity: ("Out-of-bounds read")
Fixes: cb33363d ("drm/amd/powerplay: add smu feature name support")
Fixes: 6b294793 ("drm/amd/powerplay: add smu message name support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>