Documentation: drm: Removing placeholders for generic drm properties description

These property descriptions were kept as placeholder. Removing them for simplicity.

Cc: damien.lespiau@intel.com
Cc: daniel.vetter@ffwll.ch
Cc: ville.syrjala@linux.intel.com
Signed-off-by: Sagar Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>