drm/i915/gvt: fix error return check for copy_gma_to_hva()

From commit 73dec95e ("drm/i915: Emit to ringbuffer directly"),
copy_gma_to_hva() now returns copied data length instead of 0, so
need to change error return check for that.

Note: Looks this is caused by backmerge conflict resolving, so
4.11-rc4 is not impacted as commit 73dec95e ("drm/i915: Emit to
ringbuffer directly") is not in 4.11. But need to fix this before I
can apply 4.12 stuff against drm-intel-next correctly.

Fixes: e5c1ff14

 ("Backmerge tag 'v4.11-rc4' into drm-next")
Cc: Dave Airlie <airlied@redhat.com>
Cc: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>