drm/i915/gvt: Fix a memory leak in intel_gvt_init_gtt()

It will causes memory leak, if the function setup_spt_oos() fail,
in the function intel_gvt_init_gtt(),
which allocated by get_zeroed_page() and mapped by dma_map_page().

Unmap and free the page,  after STP oos initialize fail,
it will fix this issue.

Signed-off-by: Zhou, Wenjia <zhiyuan_zhu@htc.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>