drm/amdgpu: signedness bug in amdgpu_cs_parser_init()

The "i" variable should be signed or it leads to a crash in the error
handling code.

Fixes: 1d263474

 ('drm/amdgpu: unwind properly in amdgpu_cs_parser_init()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>