gpu/doc: Fix up remaining occurrences of old document title

Following Daniel's renaming of the document.
Signed-off-by: NLukas Wunner <lukas@wunner.de>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/gpu.tmpl

@@ -2,7 +2,7 @@
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
 	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
 
-<book id="drmDevelopersGuide">
+<book id="gpuDevelopersGuide">
   <bookinfo>
     <title>Linux GPU Driver Developer's Guide</title>
 
@@ -78,9 +78,9 @@
   <title>DRM Core</title>
   <partintro>
     <para>
-      This first part of the DRM Developer's Guide documents core DRM code,
-      helper libraries for writing drivers and generic userspace interfaces
-      exposed by DRM drivers.
+      This first part of the GPU Driver Developer's Guide documents core DRM
+      code, helper libraries for writing drivers and generic userspace
+      interfaces exposed by DRM drivers.
     </para>
   </partintro>
 
@@ -3889,8 +3889,8 @@ int num_ioctls;</synopsis>
 
   <partintro>
     <para>
-      This second part of the DRM Developer's Guide documents driver code,
-      implementation details and also all the driver-specific userspace
+      This second part of the GPU Driver Developer's Guide documents driver
+      code, implementation details and also all the driver-specific userspace
       interfaces. Especially since all hardware-acceleration interfaces to
       userspace are driver specific for efficiency and other reasons these
       interfaces can be rather substantial. Hence every driver has its own