headers: Remove _DRM_GEM

This was purely a kernel-internal type used in a early patch version
to add GEM bo mmap support. It was never used in-kernel in merged code
nor in userspace. Nuke it to align with kernel headers.

For reference the kernel patch:

commit 05f51722a154e73019434bd020e50ddb941046c5
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Dec 11 11:34:32 2013 +0100

    drm/bufs: remove handling of _DRM_GEM mappings

    Gone with the new gem vma offset manager from David.

    We can also ditch the uapi header definition from the enum since
    userspace never used this. It ended up in there purely for historical
    reasons (for reusing the old drm mmap code essentially), not because
    userspace ever needed it.

    Cc: David Herrmann <dh.herrmann@gmail.com>
    Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Signed-off-by: Dave Airlie <airlied@redhat.com>

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm.h b/include/drm/drm.h
index a950b58..e213ec6 100644
--- a/include/drm/drm.h
+++ b/include/drm/drm.h
@@ -181,7 +181,6 @@
 	_DRM_AGP = 3,		  /**< AGP/GART */
 	_DRM_SCATTER_GATHER = 4,  /**< Scatter/gather memory for PCI DMA */
 	_DRM_CONSISTENT = 5,	  /**< Consistent memory for PCI DMA */
-	_DRM_GEM = 6		  /**< GEM object */
 };
 
 /**