drm/vmwgfx: Use VMW_DEBUG_USER for device command buffer errors

DRM_ERROR overwhelms dmesgi so use VMW_DEBUG_USER instead. Any malformed
command should not really go to device so WARN_ONCE to spot this.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>