int drm_open (struct inode * inode, struct file * filp);
\param inode device inode \param filp file pointer. \return zero on success or a negative number on failure.
Searches the DRM device with the same minor number, calls open_helper, and increments the device open count. If the open count was previous at zero, i.e., it's the first that the device is open, then calls setup.