drm: rename drm_fops.c to drm_file.c

It's not just file ops, but drm_file stuff in general. This is prep
work to extracting a drm_file.h header in the next patch.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-8-daniel.vetter@ffwll.ch