drm: Remove DRM_MINOR_CNT

This was originally added by David Herrmann for range checks, but
entirely unused. It confused me, so let's remove it.

Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-9-daniel.vetter@ffwll.ch