dmr/amdgpu: Fix compile error with CONFIG_DRM_AMDGPU_GART_DEBUGFS

Double defintion of 'i'

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>