Rename DRM_NODE_RENDER to DRM_NODE_PRIMARY

Now that there are render nodes it doesn't seem appropriate for the type of
the card nodes to be DRM_NODE_RENDER. For this reason, rename this type to
DRM_NODE_PRIMARY as this name better represents the purpose of these nodes.

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>