drm/doc: Put all driver docs into a separate chapter

We have quite a few driver docs now, which is great, but having them
all in the top-level gpu documentation chapter makes it harder to spot
the core/shared bits.

Stuff them into a separate chapter and ecourage people to add even
more!
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180316075926.13584-1-daniel.vetter@ffwll.ch

Documentation/gpu/drivers.rst

+========================
+GPU Driver Documentation
+========================
+
+.. toctree::
+
+   i915
+   meson
+   pl111
+   tegra
+   tinydrm
+   tve200
+   vc4
+   bridge/dw-hdmi
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/gpu/index.rst

@@ -10,16 +10,9 @@ Linux GPU Driver Developer's Guide
    drm-kms
    drm-kms-helpers
    drm-uapi
-   i915
-   meson
-   pl111
-   tegra
-   tinydrm
-   tve200
-   vc4
+   drivers
    vga-switcheroo
    vgaarbiter
-   bridge/dw-hdmi
    todo
 
 .. only::  subproject and html