drm/doc: Rename docbook to gpu.tmpl

DRM is a lot more than a direct rendering manager nowadays, and there's
also a bunch of things worth documenting for gpu driver developers
outside of drivers/gpu/drm, like vgaarb, vga_switcheroo or the various
hardware buses like host1x and ipu-v3.

To avoid further confusion let's rename the top-level to reflect
reality.

And yes I'm already looking forward to when we need to replace the G
in GPU with a * ;-)

Inspired by a thread with Lukas since he refused to include the
vga_switcheroo docs into the drm docs because it's not drm.

Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
[Lukas: Drop BUG() easter egg in i915_gem_execbuffer.c spotted by Jani
and fix typos in commit message.]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>