drm/i915: remove unnecessary null test

While creating the debugfs file we are setting the inode->i_private to
dev. That same dev is passed to these functions as private of struct
seq_file via single_open(). Moreover single_open is setting
file->private_data->private to dev.
So at this point it can never be NULL.
This check was added by commit eb3394fa ("drm/i915: Add debugfs test
control files for Displayport compliance testing")

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>