drm/amdgpu: Fix infinite loop in gfxhub_v1_2_xcc_gart_enable (v2)

An instance of for_each_inst() was not changed to match its new
behaviour and is causing a loop.

v2: remove tmp_mask variable

Fixes: b579ea63 ("drm/amdgpu: Modify for_each_inst macro")
Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>