drm: rename DRM_MINOR_LEGACY to DRM_MINOR_PRIMARY

The minor referred to by "DRM_MINOR_LEGACY" is called 'dev->primary' and
gets 'cardX' as name assigned. Lets reduce this magnificent number of
names for the same concept by one and rename DRM_MINOR_LEGACY to
DRM_MINOR_PRIMARY (to match the actual struct-member name).

Furthermore, this is in no way a legacy node, so lets not call it that.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160803180432.1341-2-dh.herrmann@gmail.com