drm/amd/powerplay: Fix error handling in smu_init_fb_allocations()

The error handling is off by one.  We should not free the first
"tables[i].bo" without decrementing "i" because that might result in a
double free.  The second problem is that when an error occurs, then the
zeroth element "tables[0].bo" isn't freed.

I had make "i" signed int for the error handling to work, so I just
updated "ret" as well as a clean up.

Fixes: f96357a9 ("drm/amd/powerplay: implement smu_init(fini)_fb_allocations function")
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>