drm: add initial DRM developer documentation

Add a DRM DocBook providing basic information about DRM interfaces, including
TTM, GEM, KMS and vblank infrastructure.  Intended to provide information to
new and existing developers about how to perform driver initialization,
implement mode setting and other DRM features.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

Documentation/DocBook/Makefile

@@ -14,7 +14,7 @@ DOCBOOKS := z8530book.xml mcabook.xml device-drivers.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
 	    mac80211.xml debugobjects.xml sh.xml regulator.xml \
 	    alsa-driver-api.xml writing-an-alsa-driver.xml \
-	    tracepoint.xml media.xml
+	    tracepoint.xml media.xml drm.xml
 
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 # The build process is as follows (targets):