drm/i915/gvt: Fix check error on hws_pga_write() fail message

Fix below check error by using proper failure message output.

drivers/gpu/drm/i915//gvt/handlers.c:1392 hws_pga_write() error: 'vgpu' dereferencing possible ERR_PTR()
drivers/gpu/drm/i915//gvt/handlers.c:1402 hws_pga_write() error: 'vgpu' dereferencing possible ERR_PTR()

Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>