drm/i915: suppress atomic commit error message under gvt-g env

drm/i915: suppress atomic commit error message under gvt-g env

under virtualization enviroment, it is possible guest update pipe
registers across vblank intervals due to overhead of mmio traps or vm
schedule out. However, it is safe since those pipe update happen in
virual registers and will not be committed to hardware. suppress that
atomic commit error message under virtualization case to avoid
confusing user.

v2: per ville's comment: return early and against Maarten's patch
v3: coding style clean

Signed-off-by: Bing Niu <bing.niu@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489004043-15449-1-git-send-email-bing.niu@intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>