drm/amdgpu: fix gart.bo pin_count leak

gmc_v{9,10}_0_gart_disable() isn't called matched with
correspoding gart_enbale function in SRIOV case. This will
lead to gart.bo pin_count leak on driver unload.

Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Leslie Shi <Yuliang.Shi@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>