drm/doc: Improve ioctl/fops docs a bit more

I spotted a markup issue, plus adding the descriptions in drm_driver.
Plus a few more links while at it.

I'm still mildly unhappy with the split between fops and ioctls, but I
still think having the ioctls in the uapi chapter makes more sense. Oh
well ...

v2: Rebase.

v3: Move misplace hunk to the right patch.

Cc: Stefan Agner <stefan@agner.ch>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170531092045.3950-1-daniel.vetter@ffwll.ch

Documentation/gpu/drm-internals.rst

@@ -198,6 +198,8 @@ drivers.
 Open/Close, File Operations and IOCTLs
 ======================================
 
+.. _drm_driver_fops:
+
 File Operations
 ---------------
 

Documentation/gpu/drm-uapi.rst

@@ -160,6 +160,8 @@ other hand, a driver requires shared state between clients which is
 visible to user-space and accessible beyond open-file boundaries, they
 cannot support render nodes.
 
+.. _drm_driver_ioctl:
+
 IOCTL Support on Device Nodes
 =============================