drm/doc: Convert to markdown

drm/doc: Convert to markdown

DRM Docbook is now Markdown ready. This means its doc is able to
use markdown text on it.

* Documentation/DocBook/drm.tmpl: Contains a table duplicated from
  drivers/gpu/drm/i915/i915_reg.h. This is not needed anymore

* drivers/gpu/drm/drm_modeset_lock.c: had a code example that used
  to look pretty bad on html. Fixed by using proper code markup.

* drivers/gpu/drm/drm_prime.c: Remove spaces between lines to make
  a proper markup list.

* drivers/gpu/drm/i915/i915_reg.h: Altought pandoc supports tables,
  it doesn't support table cell spanning. But we can use fixed-width
  for those special cases.

* include/drm/drm_vma_manager.h: Another code example that should be
  proper indented with four spaces.

v2 (Daniel): Adjust name to gpu.xml due to rename.

v3 (Daniel):
Split out the actual enabling in the Makefile - this way we can merge
the conversion, while just keeping the enabling in a drm-private tree.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk> (v1)
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephan Mueller <smueller@chronox.de>
Cc: Michal Marek <mmarek@suse.cz>
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: intel-gfx <intel-gfx@lists.freedesktop.org>
Cc: dri-devel <dri-devel@lists.freedesktop.org>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448471279-19748-2-git-send-email-daniel.vetter@ffwll.ch